Document some nodes related to pattern matching

[herwinw]

Part of #2123

This adds documentation for ArrayPatternNode, FindPatternNode, HashPatternNode, LocalVariableTargetNode (a small update) and MatchRequiredNode.

[herwinw]

MatchRequiredNode has some additional information on what types of nodes it will generate:

- name: pattern
  type: node
  comment: |
    Represents the right-hand side of the operator. The type of the node depends on the expression.

    Anything that looks like a local variable name (including `_`) will result in a `LocalVariableTargetNode`.

        foo => a # This is equivalent to writing `a = foo`
               ^

    Using an explicit `Array` or combining expressions with `,` will result in a `ArrayPatternNode`. This can be preceded by a constant.

        foo => [a]
               ^^^

        foo => a, b
               ^^^^

        foo => Bar[a, b]
               ^^^^^^^^^

    If the array pattern contains at least two wildcard matches, a `FindPatternNode` is created instead.

        foo => *, *a
               ^^^^^

    Using an explicit `Hash` or a constant with square brackets and hash keys in the square brackets will result in a `HashPatternNode`.

        foo => { a: 1, b: }

        foo => Bar[a: 1, b:]

        foo => Bar[**]

    Anything else will result in the regular node for that expression, for example a `ConstantReadNode`.

        foo => CONST

But this information applies to other locations as well. It is recursively applied to all possible fields (e.g. something that looks like a variable name inside an ArrayPatternNode will result in a LocalVariableTargetNode. And this applies to all other pattern matching related parse nodes as well (like foo in [a]). There probably is a better way to document this than to copy-paste it everywhere or referring back to other nodes for details.
Marking this one as draft until this issue has been resolved/answered.

[kddnewton]

This looks great, @herwinw is this good to go?

[kddnewton]

Oh sorry I didn't see your comment. Typically what I have done in those cases is put the documentation into the parsing_rules.md file and linked to it. Could you do the same?

[herwinw]

I will have a look at this to finish this one up, but that's going to be this weekend (or maybe later), I'm a bit short on time lately.

[kddnewton]

No problem take your time!