Just like the last commit, but now for manpages.
genMan still works with a buffer and returns []byte instead of working
directly with an io.Writer. This is because, in turn, md2man takes byte
slices instead of readers and writers.
Wrapping genMan around a writer is unnecessary especially since it's not
an exported function, and also because we'd still need a buffer to get
the output bytes.
* Move man_docs and md_docs into new doc pkg
* Replace *bytes.Buffer with io.Writer
* Replace c == cmd.helpCommand with c.IsHelpCommand()
* Remove redundant len(children) == 0 check in HasSeeAlso
* Duplicate test setup for doc generation
