Rethink Documentation Links to Parameters and Return types

[InsertCreativityHere]

We don't have a special tag to reference parameters, we re-use the same @link and @see tags you use to link to everything else. This seems like it was originally an intentional design, with the idea that the code-gen could decide whether to use paramref or <c> depending on context.

Somewhere along the way we forgot this, or changed our minds on the behavior without properly re-implementing.
On this issue, I confidently declare that:
"Parameters ... can't validly be referenced by any Slice definition, and we don't allow linking to them in doc comments."
But I can't find a single point in the repo where linking to parameters is disallowed.

We also allow you to link to named return-type elements with @link and @see as well. It's not clear if this was intentional or just a bug. The code-gen has never handled this correctly.

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Proposal

I propose we disallow pointing @link and @see tags at parameters (and return members).
In it's place, we should add a new @paramref tag.
This tag works exactly like @link, except it's:

• Only allowed on operations
• Can only link to a parameter of the operation you're documenting
  For example: @returns this returns @paramref input + 2

Alternatively, we could fix the code-gen to match the "original vision" of @link and @see working for everything,
but I don't think we should.

1. Many languages don't treat parameters as linkable types (C# and Rust certainly don't). So, there's no practical value.
2. It would add complexity to the code-generation.

[InsertCreativityHere]

We disallowed linking to parameters in #757.