Doc: Added more information about the WriteOnly mode

Task-number: QTBUG-34336

Change-Id: I4a9995dde400145cb1e33e625715eea8aaed3191
Reviewed-by: Topi Reiniö <topi.reinio@digia.com>
Doc: Remove section on how build the MySQL driver for MinGW users

The section is clearly outdated. It references MinGW from mingw.org, which
we don't support anymore. With a more modern MinGW you can just use the
recipe to link against the prebuilt binaries the same way we do describe
for MSVC.

Task-number: QTBUG-17268
Change-Id: I15b96fd328605b60d1494a703b9005e2c16aea39
Reviewed-by: Venugopal Shivashankar <venugopal.shivashankar@digia.com>
Reviewed-by: Jerome Pasion <jerome.pasion@digia.com>