Document obsolete from/toNSData functions

This patch adds the obsolete documentation for fromNSData and toNSData
to conform to the documentation process. See comments in the bug
report

Task-number: QTBUG-36985
Change-Id: I7a9b69fa99c8b53c002a95bd9c70cc64006cd59e
Reviewed-by: Jake Petroules <jake.petroules@petroules.com>
Reviewed-by: Gabriel de Dietrich <gabriel.dedietrich@digia.com>