[doc] pagegenerators: update MySQLPageGenerator
[doc] pagegenerators: update MySQLPageGenerator

https://www.mediawiki.org/wiki/Manual:Pywikibot/MySQL
help page was created recently (T187124), let's add a link to it.
Explicit the fact that two and only two columns are wanted.

Bug: T187124
Change-Id: I0555ef64df38257b265869e02bb8a503f64dfde5

wikimedia/pywikibot-core,PersianWikipedia/pywikibot-core,wikimedia/pywikibot-core