diff: Fix usage message and documentation.

The --ignore-all-space option was incorrectly documented as
--ignore-all-blanks in some (but not all) places.

MFC after: 3 days
PR: 280434
Sponsored by: Klara, Inc.
Reviewed by: 0mp, markj
Differential Revision: https://reviews.freebsd.org/D46160

(cherry picked from commit ab830de9f58c1b257e42b4fbdd46b3ca6a837ee6)