Docproc handler threadpool documentation

[johansolbakken]

This PR:

• Add documentation on configuring the thread pool for the docproc handler (see Reapply "Add ContainerThreadpool to docproc handler" vespa#35148).
• Reference this documentation in the “Container tuning” section.
• Remove the “effective default values” section from the container tuning section. The default behavior is now handled in the config model (see Refactor minThreads, maxThreads and queueSize in config-model and threadpool implementation vespa#35112).

TODO write which version it becomes available