[bug-diffutils] Stop talking about paired options like different options

[jidanni]

(info "(diff) White Space")

   The `-b' and `--ignore-space-change' options ignore white space at line

   The `-w' and `--ignore-all-space' options are stronger than `-b'.

Stop talking about them like they might be different options!

Say instead

   The `--ignore-space-change' (-b) options ignore white space at line

etc.

Else you have people combing the manual to confirm if
--ignore-space-change is -b or not. etc.

Many such pairs scattered around the manual.