NatSpec: Disallow @return tag on events.

[cuiweixie]

Description

Ethereum events do not declare return parameters, unlike functions. The event branch of callable documentation incorrectly accepted the same tag set as functions, allowing misleading @return markup. Restrict event validation to dev, notice, and param, consistent with errors.

Checklist

• I have read the contributing guidelines and the review checklist
• I have personally reviewed, understood, and tested every change in this PR

AI Disclosure

• No AI tools were used
• AI tools were used (details below)

[nikola-matic]

Before merging, this will need a few additions:

1. a natspecJSON isoltest, something like:

  contract C {
      /// @return foo
      event E(uint x);
  }

2. Same as above, but for inherited events (in general, see test/libsolidity/natspecJSON/invalid/* for examples)
3. It will also need a changelog entry (see previous Natspec changelog entries for inspiration).

[nikola-matic]

This is the last set of changes - otherwise looks good. Please make the changes so we can merge.

[nikola-matic]

Please squash the last two commits into the first one, and leave the changelog one as separate, i.e.

Update test/libsolidity/natspecJSON/invalid/docstring_event_return_inheritance.sol -> NatSpec: Disallow @return tag on events.
fix: review -> NatSpec: Disallow @return tag on events.