Documenting every AST node's identity macro interpolation

[HertzDevil]

#14063 added the following documentation to Crystal::Macros::ModuleDef:

Every module definition node is equivalent to:

{% begin %}
  {{ node.kind }} {{ node.name }}
    {{ node.body }}
  end
{% end %}

This is an identity macro interpolation; the resulting Crystal code parses to the same object as node. Having those identities in the documentation is handy when writing macros that transform nodes, such as adding parameters to a Def or a Call's block. They are also a motivation behind #3274, in that every composite AST node should be expressible as an interpolation using its components. We should document those identities for all the other Crystal::Macros types.

There is in fact a long form of the above identity, which further decomposes node.name but without going through that temporary node:

{% begin %}
  {% type_vars = node.type_vars %}
  {{ node.kind }} {{ node.name(generic_args: false) }}{% unless type_vars.empty? %}(
    {% for var, i in type_vars %}
      {% if i == node.splat_index %}*{% end %}{{ var }},
    {% end %}
  ){% end %}
    {{ node.body }}
  end
{% end %}

When an AST node has both a full decomposition like above, and a convenience identity interpolation (e.g. Def after #13082), we should probably document both forms.