docs/completion.txt: Fix a couple of punctuation nits

This patch fixes a couple of punctuation nits which can make the document
more correct and readable.

Also missing "()" are added to some function references for consistency.

Signed-off-by: John Garry <john.garry@huawei.com>
Acked-by: Randy Dunlap <rdunlap@infradead.org>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>