bug#46979: Allow setting Man-default-entry via Local Variables

[Eli Zaretskii]

> From: 積丹尼 Dan Jacobson <jidanni@jidanni.org>
> Cc: mardani29@yahoo.es,  46979@debbugs.gnu.org
> Date: Mon, 08 Mar 2021 23:51:03 +0800
> 
> E.g., we are editing our crontab file, and do M-x man
> we don't want to have to type in c r o n t a b ( 5 ) each time.

Who on earth needs to read the man page of crontab(5)?

Any other examples?