[7.1.0] Document --cache_computed_file_digests. (#21326)

It seems pretty important to document the existence of this flag, as it
has implications for both performance and correctness.

See https://github.com/bazelbuild/bazel/issues/14401 for prior
discussion.

PiperOrigin-RevId: 606545000
Change-Id: I3aa9d311c805ebf9fb7401b79f320e1ba11d2179