API: Fix action=parse without any page or title or text

action=parse was raising an error about missing 'text' with 'title' even
when 'title' wasn't actually passed, due to a fix for bug 33865. But
this broke using action=parse to parse an edit summary without also
giving it wikitext to parse.

Instead, let's give the client a warning (unless it seems clear they
know what they are doing) and assume the empty string for 'text'.

Also, while we're editing the file, add some more examples as also
requested in bug 48319.

Bug: 48319
Change-Id: I03c1fbcb0bd31dea8bd84e164104f7ced0ace449