API i18n: Fix consistency in messages

Fixed consistency in two messages and added article to a third.
Affected messages:
* apihelp-visualeditor-description
* apihelp-visualeditor-param-oldid
* apihelp-visualeditoredit-param-oldid

Reported at:
https://translatewiki.net/wiki/MediaWiki:Apihelp-visualeditor-param-oldid/sv
https://translatewiki.net/wiki/MediaWiki:Apihelp-visualeditoredit-param-oldid/sv

Change-Id: I054cf021482276c3676e91d9cf9f6fcdfc12d583