TRY300: Add some extra notes on not catching exceptions you didn't expect

[wlach]

Summary

Added some extra notes on why you should have focused try...except blocks to TRY300.

When fixing a violation of this rule, a co-worker of mine (very understandably) asked why this was better. The current docs just say putting the return in the else is "more explicit", but if you look at the linked reference in the python documentation they are more clear on why violations like this is bad:

The use of the else clause is better than adding additional code to the try clause because it avoids accidentally catching an exception that wasn’t raised by the code being protected by the try … except statement.

This is my attempt at adding more context to the docs on this. Open to suggestions for wording!

Test Plan

N/A, just docs

[github-actions]

ruff-ecosystem results

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