Document the recursion_limit built-in attribute

[kiana1kaslana]

Document the recursion_limit built-in attribute in std via
#[doc(attribute = "recursion_limit")], covering:

• what the limit applies to (macro expansion, auto-dereference)
• the default (128) and how to raise it
• the diagnostics produced when the limit is reached
• a note on when raising it is appropriate

As claimed in #157604 (comment).

Part of #157604.

[rustbot]

Thanks for the pull request, and welcome! The Rust Project has assigned @JohnTitor (or someone else) to review your changes, you should hear from them (or someone else) within the next two weeks.

Please see the contribution instructions and our LLM policy for more information.

Why was this reviewer chosen?

The reviewer was selected based on:

• Owners of files modified in this PR: libs
• libs expanded to 12 candidates
• Random selection from Darksonn, JohnTitor, Mark-Simulacrum, clarfonthey

[lukaslueg]

Personal drive-by suggestions: While the docs do introduce "compile-time operations" explicitly, I suggest to go one step further add a clarifying sentence wrt not having anything to do with run-time recursion; people have confused the attribute to affect recursive code, which it does not. Also I suggest to explicitly advise readers to increase the limit in measured steps instead of just punching in 9999999; punching the limit may cause other people to experience resource exhaustion and general sadness due to what in their case is an unlimited infinite recursion.

[kiana1kaslana]

Personal drive-by suggestions: While the docs do introduce "compile-time operations" explicitly, I suggest to go one step further add a clarifying sentence wrt not having anything to do with run-time recursion; people have confused the attribute to affect recursive code, which it does not. Also I suggest to explicitly advise readers to increase the limit in measured steps instead of just punching in 9999999; punching the limit may cause other people to experience resource exhaustion and general sadness due to what in their case is an unlimited infinite recursion.

Good points, thanks. I added a clarifying sentence that the limit is compile-time only and has no effect on runtime recursion, and I explicitly advise raising it in measured steps instead of jumping to a very large number.