As hoped, things calmed down for docs this cycle; fewer changes and almost

no conflicts at all.  This pull includes:

 - A reworked and expanded user-mode Linux document
 - Some simplifications and improvements for submitting-patches.rst
 - An emergency fix for (some) problems with Sphinx 3.x
 - Some welcome automarkup improvements to automatically generate
   cross-references to struct definitions and other documents
 - The usual collection of translation updates, typo fixes, etc.
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