rfc: update error message for misplaced doc comments

Stream: lean4

[Alok Singh]

GPT often puts doc comments in inapropos spots (namespace, section, import, open) but even I've done it and scratched my head a second figuring out the error since it gives a rather long and a bit crytpic parse error:

unexpected token 'namespace'; expected '#guard_msgs', 'abbrev', 'add_decl_doc', 'axiom', 'binder_predicate', 'builtin_dsimproc', 'builtin_dsimproc_decl', 'builtin_grind_propagator', 'builtin_initialize', 'builtin_simproc', 'builtin_simproc_decl', 'class', 'declare_simp_like_tactic', 'declare_syntax_cat', 'def', 'dsimproc', 'dsimproc_decl', 'elab', 'elab_rules', 'example', 'grind_propagator', 'inductive', 'infix', 'infixl', 'infixr', 'initialize', 'instance', 'macro', 'macro_rules', 'notation', 'opaque', 'postfix', 'prefix', 'recommended_spelling', 'register_tactic_tag', 'simproc', 'simproc_decl', 'structure', 'syntax', 'tactic_extension', 'theorem' or 'unif_hint'

I propose that the error message reflect that doc comments can't be put on that item, and specifically mentions doc comments in the error message for benefit of man and machine.

[Joachim Breitner]

(JFTR: Open issue for that: https://github.com/leanprover/lean4/issues/7591)