Explain doclets better

[hegemonic]

JSDoc uses the term "doclet" to refer to an object that represents the documentation for a JS symbol. It's important to know something about doclets if you're doing any of the following:

• Creating or modifying a JSDoc template
• Creating or modifying a JSDoc plugin
• Using JSDoc to create a JSON file, then feeding the JSON file to another tool

We need a page that does all of the following:

• Explains what a doclet is
• Shows the typical properties in a doclet
• Lists the non-enumerable properties in a doclet, and shows examples of each property
• Points readers to the JSON schema for doclets

[hegemonic]

Folding #58 into this issue:

Starting with jsdoc/jsdoc@1171593, doclets can contain a parsedType property that contains the parsed type from Catharsis. The contents of this property are subject to change.

The property is non-enumerable unless the --debug flag is enabled.