docs/manual/: Replace one diagram with two separate ones and updates others.

docs/manual/: Replace one diagram with two separate ones and updates others.

Original commit message from CVS:
patch by: Luc Pionchon <luc.pionchon@nokia.com>
* docs/manual/advanced-threads.xml:
* docs/manual/diagrams-pipelines.svg:
* docs/manual/hello-world.png:
* docs/manual/linked-elements.png:
* docs/manual/mime-world.png:
* docs/manual/queue.png:
* docs/manual/thread-buffering.png:
* docs/manual/thread-synchronizing.png:
Replace one diagram with two separate ones and updates others.
Fixes #542401.