docs/manual/: Rewrites. Remove cothreads, go a bit into opt specifically, document...

docs/manual/: Rewrites. Remove cothreads, go a bit into opt specifically, document threads and their gotchas, and do ...

Original commit message from CVS:
* docs/manual/advanced-autoplugging.xml:
* docs/manual/advanced-schedulers.xml:
* docs/manual/advanced-threads.xml:
Rewrites. Remove cothreads, go a bit into opt specifically,
document threads and their gotchas, and do some technical stuff
on autoplugging plus add some working examples. Fixes #157395.
* examples/manual/Makefile.am:
Add typefind/autoplugger example (one that actually works).
Remove queue example since it's a duplicate of the thread one.