Change proc_macro::Diagnostics docs

[tsoutsman]

With Rust 1.54 attributes can invoke function-like macros. This allows functions generated by macros to have non-generic documentation. This pull request changes the documentation of proc_macro::Diagnostics to be more specific and include a link to the level.

Example

Before:

Adds a new child diagnostic message to `self` with the level
identified by this method’s name with the given `message`.

After:

Adds a new child diagnostic message to self with the [`Level::Error`]
level, and the given `message`.

[rust-highfive]

Thanks for the pull request, and welcome! The Rust team is excited to review your changes, and you should hear from @petrochenkov (or someone else) soon.

Please see the contribution instructions for more information.

[petrochenkov]

Could you wrap the lines to fit into the usual 100-character limit? rustfmt cannot do it automatically in this case.

r=me after doing that and squashing commits.

[tsoutsman]

Hi, I'm not quite sure about the indentation so I just put it in line with concat!
r= @petrochenkov

[petrochenkov]

Thanks!
@bors r+

[bors]

📌 Commit 75e8035 has been approved by petrochenkov

[petrochenkov]

@tsoutsman
Push to a pull request after approval (@bors r+) cancels the approval.
Also using rebases instead of merges is recommended to update pull request branches.

[petrochenkov]

@bors r+

[bors]

📌 Commit 26bf0ef has been approved by petrochenkov

[tsoutsman]

@tsoutsman
Push to a pull request after approval (@bors r+) cancels the approval.
Also using rebases instead of merges is recommended to update pull request branches.

Sorry, I was just playing around with the repository. I'm not too familiar with git.

[bors]

⌛ Testing commit 26bf0ef with merge e8c25f2...

[bors]

☀️ Test successful - checks-actions
Approved by: petrochenkov
Pushing e8c25f2 to master...