QDoc: Clarify raw-markup guidance

The raw-command manual section suggests working around missing table
or list presentation with raw markup. That advice encourages
format-specific HTML in authored documentation.

Recommend QDoc markup for new content while explaining that existing
raw blocks still render for their selected format.

Pick-to: 6.12
Change-Id: Id0962ea1bee8c8688c5622d98df62f9cb3bd7078
Reviewed-by: Topi Reinio <topi.reinio@qt.io>