man: use title of docs/ pages when referring to them

man: use title of docs/ pages when referring to them

There is some inconsistency, partially caused by the awkward naming
of the docs/ pages. But let's be consistent and use the "official" title.
If we ever change plural↔singular, we should use the same form everywhere.

(cherry picked from commit d6029680df7c4991e37662467668816a83c0b806)
(cherry picked from commit 77681242c8c6d7693814b8245e9096e43faa21be)