Logging: Mention 'best practices' for using QLoggingCategory

QLoggingCategory objects are meant to be mere 'handles' for the
registry. It's therefore not recommended to
- manipulate them directly (via setEnabled()), except in a filter
- export them across module boundaries
- subclass them

Subclassing QLoggingCategory also breaks compilations in a certain
circumstances (no variadic macros).

Task-number: QTBUG-37283
Change-Id: Ib12fb43d955902c7fa4583296d64afc5eca01200
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>