Add a doc about the Custom Javadoc Tags feature (#1085)

* Add a doc about the Custom Javadoc Tags feature

* Format content

* Add pictures

Author: onewhl

docs/images/utbot_custom_javadoc_tags.png

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+## Custom Javadoc Tags
+
+Currently, summaries are hard to read because of formatting issues and a lot of details they contain.
+
+**Goal**: to make test summaries structured and clear.
+
+**Idea**: to structure summary by introducing custom JavaDoc tags.
+
+Tags should start with a prefix "utbot" to avoid possible issues with user (or some plugin) custom tags (if they have
+the same names).
+
+**Suggested custom tags (NOT ALL OF THEM ARE USED)**
+
+| Name                       | Description                        | Usage example                                                   |
+|----------------------------|------------------------------------|-----------------------------------------------------------------|
+| `@utbot.classUnderTest`    | Inline link to the enclosing class | `@utbot.methodUnderTest {@link main.IntMath}`                   |     
+| `@utbot.methodUnderTest`   | Inline link to the method we test. | `@utbot.methodUnderTest {@link main.IntMath#pow(int, int)}`     |
+| `@utbot.expectedResult`    | Value we expect to get.            | `@utbot.expectedResult 4`                                       |
+| `@utbot.actualResult`      | Value we got.                      | `@utbot.actualResult 64`                                        |
+| `@utbot.executes`          | Executed condition.                | `@utbot.executes {@code (k < Integer.SIZE): True}`              |
+| `@utbot.invokes`           | Invoked method.                    | `@utbot.invokes {@link main.IntMath#mul(int, int)}`             |
+| `@utbot.triggersRecursion` | Triggered recursion.               | `@utbot.recursion triggers recursion of leftBinSearch`          |
+| `@utbot.activatesSwitch`   | Activated switch case.             | `@utbot.activatesSwitch {code case 10}`                         |
+| `@utbot.returnsFrom`       | Statement we return from.          | `@utbot.returnsFrom {@code return (k == 0) ? 1 : 0;}`           |
+| `@utbot.throwsException`   | Thrown exception.                  | `@utbot.throwsException {@link java.lang.NullPointerException}` |
+| `@utbot.caughtException`   | Caught exception.                  | `@utbot.caughtException {@code RuntimeException e}`             |
+
+## Settings
+
+There is a setting `Javadoc comment style` in the main plugin's `Settings`. It has two options: `Plain` text
+and `Structured via custom Javadoc tags` (selected by default).
+
+![Settings](../images/utbot_settings.png)
+
+## View
+
+There are two modes the comment could be shown in IntelliJ IDEA: plain text and rendered view.
+
+To activate rendered mode, click on the toggle near comment.
+
+![Example](../images/utbot_custom_javadoc_tags.png)
+
+## Implementation details
+
+Implemented `JavadocTagInfo` to introduce our custom JavaDoc tags.
+
+Implemented `CustomJavadocTagProvider` and registered it in `plugin.xml` to support plugin's custom tags.
+
+Overrided behavior of `JavaDocumentationProvider#generateRenderedDoc` and registered it in `plugin.xml` to render our
+custom JavaDoc tags correctly.
+
+Added a flag `USE_CUSTOM_TAGS` to settings.
+
+After plugin's removal, IDE doesn't recognize our custom tags. It doesn't lead to errors, but highlights tags with
+yellow color.
+
+## Feedback
+
+We held a Feature Demo meeting to gather some feedback (16.08.2022).
+
+In general, teammates said that the idea and the way it looks like is good and suggested several things to do:
+
+- Investigate if it's possible to add a link to menu settings or file with stacktrace in comments.
+- There is no need in classUnderTest tag because methodUnderTest contains this information.
+- Add a tag describing the test intention: check boundary cases, calls of some interesting methods.
+- Prepare survey and ask Artem Aliev's team to try the feature and share feedback.
+
+## Test scenarios
+
+Currently, the feature works only for Symbolic execution engine, so make sure the slider is on the Symbolic execution
+side.
+
+### Default behaviour (the feature is enabled).
+
+1. Run plugin on any Java project and run tests generation.
+2. Check if the comments are generated and pretty formatted.
+3. Check that all links are clickable (parts that start with `@link` tag).
+
+### Manual settings (you can choose any comment style – old and new).
+
+1. Go to the `Settings` menu, check that the drop-down list `Javadoc comment style` exists and has two options (Plain
+   text
+   and Structured via custom Javadoc tags).
+2. Select any option, click OK, run tests generation and check that the option is applied and the comments are generated
+   according to the chosen style.
+
+### Content
+
+1. First, generate comment with one style, then generate with another one and compare its content. If it differs,
+   please, provide code snippet and both generated comments. It could differ because currently the
+   style with custom Javadoc tags is a bit simplified.
+
+### View
+
+1. Check that the comments are rendered well. To do it, click on the toggle near the comment (see post about Rendered
+   view feature in IntelliJ IDEA).