Doc: Update, and reduce duplication of, QThread-related info

Doc: Update, and reduce duplication of, QThread-related info

Added/Changed:
- Move content from the Thread Basics overview to the QThread class ref
- Rephrase bits for clarity
- Use more links

Removed:
- (threads-basics.qdoc) Warning against moveToThread(this): This usage
  came about when people tried to add slots to a QThread subclass. This
  patch adds a warning against the root cause.
- (threads-basics.qdoc) The strategy for managing member variables:
  Sounds error-prone. Pushing results through signals is safer.
- (qthread.cpp) The note about GUI classes: Irrelevant to QThread,
  and it's already mentioned elsewhere.

This is a cherry-pick from a9d5627e6a7b82 in qtbase.git.

Change-Id: I491f64f998050daf0251abb2126bc9f7a198c17d
Reviewed-by: Sze Howe Koh <szehowe.koh@gmail.com>
Reviewed-by: Jerome Pasion <jerome.pasion@digia.com>