docs/184791: [patch] man page for 'diff3' references 'info diff' instead of 'info diff3'

[Warren Block]

On Sun, 5 Jan 2014, Jamie Landeg Jones wrote:

>>
>> Normally, it would come in when the local diffutils are updated to the
>> latest upstream version.  I don't know when that will be or who would
>> handle it.
>
> Yeah, that's what I meant. I'd be willing to have a look at it myself
> if I knew they were open to submissions!

I looked a bit more.  We currently have a fairly old version of 
diffutils, 2.8.7.  It's unlikely that newer versions will be imported 
because they are GPL: https://wiki.freebsd.org/GPLinBase

Instead, there is a project to import the OpenBSD versions:
https://wiki.freebsd.org/SummerOfCode2012/JesseHagewood
Current status is not clear.

Given that, I don't see why we can't make interim changes to the GNU 
versions present now.  (And I'm sorry I didn't look at that earlier.)

I'll take this and commit it to -CURRENT, with an MFC planned to 
10-STABLE and 9-STABLE.