docs: Fix public_overrides module structure

Update the C++ public override headers and Multiple Facades sections in
the module structure documentation to specify that public override
headers must go in public_overrides/<build_target_name>/<path>/*.h
rather than <build_target_name>_public_overrides/<path>/*.h.

Change-Id: I5ed7473580c991946cf68163934b13d086b7d763
Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/pigweed/+/486130