docs: multi-document README; fixes #365 ; fixes #886

Author: brettz9

.README/README.md

@@ -0,0 +1,88 @@
+## Advanced
+
+{"gitdown": "contents", "maxLevel": 6, "rootId": "advanced"}
+
+### AST and Selectors
+
+For various rules, one can add to the environments to which the rule applies
+by using the `contexts` option.
+
+This option works with [ESLint's selectors](https://eslint.org/docs/developer-guide/selectors) which are [esquery](https://github.com/estools/esquery/#readme)
+expressions one may use to target a specific node type or types, including
+subsets of the type(s) such as nodes with certain children or attributes.
+
+These expressions are used within ESLint plugins to find those parts of
+your files' code which are of interest to check. However, in
+`eslint-plugin-jsdoc`, we also allow you to use these selectors to define
+additional contexts where you wish our own rules to be applied.
+
+#### `contexts` format
+
+While at their simplest, these can be an array of string selectors, one can
+also supply an object with `context` (in place of the string) and one of two
+properties:
+
+1. For `require-jsdoc`, there are also `inlineCommentBlock` and
+    `minLineCount` properties. See that rule for details.
+1. For `no-missing-syntax` and `no-restricted-syntax`, there is also a
+    `message` property which allows customization of the message to be shown
+    when the rule is triggered.
+1. For `no-missing-syntax`, there is also a `minimum` property. See that rule.
+1. For other rules, there is a `comment` property which adds to the `context`
+    in requiring that the `comment` AST condition is also met, e.g., to
+    require that certain tags are present and/or or types and type operators
+    are in use. Note that this AST (either for `Jsdoc*` or `JsdocType*` AST)
+    has not been standardized and should be considered experimental.
+    Note that this property might also become obsolete if parsers begin to
+    include JSDoc-structured AST. A
+    [parser](https://github.com/brettz9/jsdoc-eslint-parser/) is available
+    which aims to support comment AST as
+    a first class citizen where comment/comment types can be used anywhere
+    within a normal AST selector but this should only be considered
+    experimental. When using such a parser, you need not use `comment` and
+    can just use a plain string context. The determination of the node on
+    which the comment is attached is based more on actual location than
+    semantics (e.g., it will be attached to a `VariableDeclaration` if above
+    that rather than to the `FunctionExpression` it is fundamentally
+    describing). See
+    [@es-joy/jsdoccomment](https://github.com/es-joy/jsdoccomment)
+    for the precise structure of the comment (and comment type) nodes.
+
+#### Discovering available AST definitions
+
+To know all of the AST definitions one may target, it will depend on the
+[parser](https://eslint.org/docs/user-guide/configuring#specifying-parser)
+you are using with ESLint (e.g., `espree` is the default parser for ESLint,
+and this follows [EStree AST](https://github.com/estree/estree) but
+to support the the latest experimental features of JavaScript, one may use
+`@babel/eslint-parser` or to be able to have one's rules (including JSDoc rules)
+apply to TypeScript, one may use `@typescript-eslint/parser`, etc.
+
+So you can look up a particular parser to see its rules, e.g., browse through
+the [ESTree docs](https://github.com/estree/estree) as used by Espree or see
+ESLint's [overview of the structure of AST](https://eslint.org/docs/developer-guide/working-with-custom-parsers#the-ast-specification).
+
+However, it can sometimes be even more helpful to get an idea of AST by just
+providing some of your JavaScript to the wonderful
+[AST Explorer](https://astexplorer.net/) tool and see what AST is built out
+of your code. You can set the tool to the specific parser which you are using.
+
+#### Uses/Tips for AST
+
+And if you wish to introspect on the AST of code within your projects, you can
+use [eslint-plugin-query](https://github.com/brettz9/eslint-plugin-query).
+Though it also works as a plugin, you can use it with its own CLI, e.g.,
+to search your files for matching esquery selectors, optionally showing
+it as AST JSON.
+
+Tip: If you want to more deeply understand not just the resulting AST tree
+structures for any given code but also the syntax for esquery selectors so
+that you can, for example, find only those nodes with a child of a certain
+type, you can set the "Transform" feature to ESLint and test out
+esquery selectors in place of the selector expression (e.g., replace
+`'VariableDeclaration > VariableDeclarator > Identifier[name="someVar"]'` as
+we have
+[here](https://astexplorer.net/#/gist/71a93130c19599d6f197bddb29c13a59/latest))
+to the selector you wish so as to get messages reported in the bottom right
+pane which match your [esquery](https://github.com/estools/esquery/#readme)
+selector).

.README/advanced.md

@@ -1,6 +1,7 @@
 ### `sort-tags`
 
-Sorts tags by a specified sequence according to tag name.
+Sorts tags by a specified sequence according to tag name, optionally
+adding line breaks between tag groups.
 
 (Default order originally inspired by [`@homer0/prettier-plugin-jsdoc`](https://github.com/homer0/packages/tree/main/packages/public/prettier-plugin-jsdoc).)