[7.3.0] Document source of truth for `rctx.download*` cache key (#23048)

Documenting the default caching behaviour for `rctx.download` and
`rctx.download_and_extract`.
 
Related to #22652

Closes #22716.

PiperOrigin-RevId: 653878029
Change-Id: I129f9eede2b841483a98d5335651c1d1198aaf76

Commit
https://github.com/bazelbuild/bazel/commit/d4b416f503d9e202dec0b24905366109ace43165



Co-authored-by: Jordan Mele <jordan.mele@outlook.com.au>