doc: Minor group/module page changes.

These changes were made in conjunction with the
large update to qdoc to teach it to use multiple
trees.

Task-number: QTBUG-35377
Change-Id: If16bfdf9186483c0b313c999c61fa4dc5cf49048
Reviewed-by: Topi Reiniö <topi.reinio@digia.com>