docs: Makefile: honor V=0 for docs building

Reduce the number of displayed mesages when building the
docs with V=0.

Suggested-by: Stephen Rothwell <sfr@canb.auug.org.au>
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>
Link: https://lore.kernel.org/r/478c114a2399b68a18de94ee5f98649304f3903b.1603796153.git.mchehab+huawei@kernel.org


Signed-off-by: Jonathan Corbet <corbet@lwn.net>