docs: document flash attention supports_mapping

[nightcityblade]

What does this PR do?

Documents the supports_mapping argument in _process_flash_attention_kwargs and fixes the docstring section heading from Return to Returns.

Fixes #45995

Before submitting

• This PR fixes a typo or improves the docs (you can dismiss the other checks if that's the case).
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  Pull Request section?
• Was this discussed/approved via a Github issue or the forum? Please add a link
  to it if that's the case. Issue: docs: Add missing parameter documentation #45995
• Did you make sure to update the documentation with your changes? Here are the
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  here are tips on formatting docstrings.
• Did you write any new necessary tests? No new tests were needed for this docstring-only change; ran python3 -m py_compile src/transformers/modeling_flash_attention_utils.py and python3 -m ruff check src/transformers/modeling_flash_attention_utils.py.

Who can review?

Documentation: @stevhliu
Attention: @vasqu @ArthurZucker @Cyrilvallez

[stevhliu]

thanks!

[stevhliu]

just specifying the wording a bit more :)

Suggested change

	Mapping indicating whether each flash attention feature is supported by the active backend.
	A dict containing the underlying information of the Flash Attention function on its supported parameters. Keys are all valid parameters (and alternative names) across all standards and values indicate whether each parameter could be found in the active backend's signature. For more information, please check `_lazy_define_process_function` that creates this mapping.

[vasqu]

Can we take a similar description to #45153?

[stevhliu]

updated to match the existing description