Bug 1332373: Add 19-character dbprefix limit to documentation

Based on the maximum database object name length in Postgres,
and the longest names of objects we're currently using, 19
characters is the hard limit. (Technically... 25 characters
for MySQL. But why confuse things?)

We're actively testing on test.mahara.org with a 19-character
db prefix, so I think it's safe to tell the users that they
can use one that long.

Change-Id: Id13ee82ca70780791870782b8465695366cee8eb
behatnotneeded: comment change only