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.
This code was already using io.Writer, but was completely ignoring write
errors.
The most worrying part is how GenMarkdownTreeCustom used an unnecessary
buffer to then dump all of its contents on a file, and instead of
returning an error on file creation/writing, it would just exit the
entire program.
* 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
