[bug-diffutils] bug#38779: Document conflicting options better

[積丹尼 Dan Jacobson]

$ diff -y --suppress-common-lines -c --show-function-line=mirah A B
diff: conflicting output style options
diff: Try 'diff --help' for more information.

Actually diff --help won't tell the user about conflicts.

One cannot even find mention in the Info manual for this case (or others.)

Also the message doesn't say which options are the conflicting ones.

diff (GNU diffutils) 3.7