[clang][analyzer][NFC] Add test for a limitation of...

[clang][analyzer][NFC] Add test for a limitation of alpha.unix.BlockInCriticalSection checker (#93799)

Updated the documentation in `checkers.rst` to include an example of how
`trylock` function is handled.
Added a new test for a scenario where `pthread_mutex_trylock` is used,
demonstrating the current limitation.