trivial: Add note about multiple builders support

Sphinx 1.6 will support the definition of multiple builders in a
setup.cfg file like so:

  [build_sphinx]
  builder = html man

Once we support this version of Sphinx, we should stop carrying the
custom versions of this tooling we use.

Upstreaming things FTW.

Change-Id: Ibf2a003229a4585df96b09da7ca547e201c5aef5

pbr/builddoc.py

@@ -183,6 +183,9 @@ class LocalBuildDoc(setup_command.BuildDoc):
                         "autodoc_exclude_modules",
                         [None, ""])[1].split()))
 
+        # TODO(stephenfin): Deprecate this functionality once we depend on
+        # Sphinx 1.6, which includes a similar feature, in g-r
+        # https://github.com/sphinx-doc/sphinx/pull/3476
         for builder in self.builders:
             self.builder = builder
             self.finalize_options()