doc(sources): Document rationale for Sentry list_file behavior

[loewenheim]

The debug files endpoint in Sentry supports querying by a list of file formats:
https://github.com/getsentry/sentry/blob/e23668f702957cc09d47977cacfa2b9c8b384928/src/sentry/api/endpoints/debug_files.py#L248

https://github.com/getsentry/sentry/blob/e23668f702957cc09d47977cacfa2b9c8b384928/src/sentry/api/endpoints/debug_files.py#L281-L287

Therefore we can already do the filtering by file type when we make the query to the Sentry source, instead of getting all files and filtering afterwards.

[Swatinem]

this was done intentionally to improve cache efficiency of the lookup cache, as well as reduce the outgoing requests to Sentry.

I believe I even wrote a shipped email about it because it turned out to be a very visible improvement.

[loewenheim]

I've changed the PR to instead add a note about why it works the way it does.