Remove an obsolete note and move the description of the
-developer-build option to its own section, now that it
is not mentioned in the topic about -prefix builds any more.

Fixes: QTBUG-63017
Change-Id: Icd4f4477278990bb76b8862a6c0b5632090095bf
Reviewed-by: Paul Wicking <paul.wicking@qt.io>