Clarify the message visualeditor-dialog-meta-categories-sortkey-label

Explaining that the sorting is about the name of the page
should make this feature clearer for new editors.

Change-Id: I9f152e0d3dc5e9bf937d9222d95b7c3c15db87f9
Merge "Clarify the message visualeditor-dialog-meta-categories-sortkey-label"