[3.10] gh-97731: Specify the full path to the docs for `make docclean` (GH-98982) (#99025)

Specify the full path to the docs for `make docclean`
This is to have `make clean` not error out on cross-builds.
(cherry picked from commit 22bab74c8f5fa6d53ddb7e40baa4334d09331c0f)