docs: format, grammarize, and standardize the wall man page

Signed-off-by: Benno Schulenberg <bensberg@justemail.net>
docs: standardize the phrases for --help and --version in all man pages

Signed-off-by: Benno Schulenberg <bensberg@justemail.net>
docs: tweak wording and formatting of unshare and nsenter man pages

Signed-off-by: Benno Schulenberg <bensberg@justemail.net>
docs: tweak formatting and grammar of the ipcrm man page

Signed-off-by: Benno Schulenberg <bensberg@justemail.net>