docs: kernel-doc: update commands to generate man page

(1) The command to generate man pages is truncated in the pdf version
of the document.  Reformat the command into multiple lines to prevent
the truncation.

(2) Older versions of git do not support all variants of pathspec
syntax.  Provide commands to generate man pages for various
alternate syntax.

Signed-off-by: Frank Rowand <frank.rowand@sony.com>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>