[7.1.0] Document `--digest_function` startup flag (#20864)

This flag is in use in production today (e.g. when using `blake3`).

Closes #20747.

Commit
https://github.com/bazelbuild/bazel/commit/6d302147eb7b2d2dc64fb92187f30591caf275ae



PiperOrigin-RevId: 597704594
Change-Id: I92b5cfc4bf70c86d50049c6881f7b5f4fb7d30be

Co-authored-by: Brentley Jones <github@brentleyjones.com>