WikiPage: Allow replaceSection on an nonexistent page

This corrects a regression, where replaceSection would require that the
page have at least one revision in the database.  If the page does not
exist, continue with the replacement, using a null baseRevId.

Also: clean up a few comments in the replaceSection functions.

Bug: 66141
Change-Id: Ieb2cb05aee07a1d8bbdc3bdfe9536dfa8bee529e
Merge "WikiPage: Allow replaceSection on an nonexistent page"