Add doxy-spellchecker info to Travis job details

Apply changes from PR #922 to 5.11 to add Travis spellchecker details.

Author: Amanda Butler

docs/reference/contributing/guidelines/ci.md

@@ -16,6 +16,12 @@ Travis configuration is located in the [.travis.yml](https://github.com/ARMmbed/
    - Assert that the Doxygen build produces no warnings.
    - Assert that all binary libraries are named correctly.
    - Assert that all assembler files are named correctly.
+- **travis-ci/doxy-spellcheck** - Check Doxygen comments for spelling errors. Runs on header files in:
+   - Drivers.
+   - Platform.
+   - RTOS.
+   - Events.
+   - Features/netsocket.
 - **travis-ci/events** - Check that Mbed OS compiles and run events tests.
 - **travis-ci/gitattributestest** - Check there are no changes after clone. This checks that `.gitattributes` is used correctly.
 - **travis-ci/licence_check** - Check there is no GPL licence text in the code.