[Bug preprocessor/91025] cpp man page incorrectly describes -MD option

[srk31 at srcf dot ucam.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=91025

--- Comment #2 from Stephen Kell <srk31 at srcf dot ucam.org> ---
Created attachment 47405
  --> https://gcc.gnu.org/bugzilla/attachment.cgi?id=47405&action=edit
Patch to cpp manual page's description of -MD

It took me long enough, but I'm fairly sure this patch results in a less-wrong
cpp man page than the current one.