Doc: Clarify status of MNG, Jpeg2000 plugins

While the repository still contains bundled copies of 3rd party codecs
for these formats, they are not part of the build process anymore;
the corresponding plugins are built only when the system libraries
are found.

Change-Id: Ib658b921bd08b63e68886922fc6bc45c5c3cc637
Reviewed-by: Leena Miettinen <riitta-leena.miettinen@qt.io>
Reviewed-by: Eirik Aavitsland <eirik.aavitsland@qt.io>