docs: Use @ref instead of @xref

docs: Use @ref instead of @xref

The @xref command is meant to be used at the beginning of a sentence
because its expansion creates a "See " prefix on all output formats, and
on older makeinfo versions is strict about enforcing a "." or "," after
the command. The @ref command has no such restriction and is just the
link, which allows more control over output. This also fixes an issue
where there was a repeated "see" in the output.

Reported-by: Olaf Hering <olaf@aepfle.de>
Signed-off-by: Glenn Washburn <development@efficientek.com>
Reviewed-by: Daniel Kiper <daniel.kiper@oracle.com>
Tested-by: Olaf Hering <olaf@aepfle.de>