man: enable warning msgs for "all" target

man: enable warning msgs for "all" target

* and remove `local-check` target. because we have killed all warnings,
  and `make check` depends on `am-all` actually. so previously, `make check`
  basically does nothing at all for validating the man pages' syntax.
  `make all` has rendered them already.
* do not run sphinx-build if $(top_srcdir)/doc/man does not exist. this
  is exactly the case of `make install` from a dist tarball.
* remove generated man pages in `make maintainer-clean` not in `make
  clean`, because we don't ship .rst files in the dist tarball.

Signed-off-by: Kefu Chai <kchai@redhat.com>