6993: Clean up descriptions for settings r=matklad a=rherrmann
Use two consecutive newlines (`\n\n`) to actually continue text on a
new line.
Use proper markup to reference related settings.
Consistently format references to files, command line arguments, etc.
as `code`. Format mentions of UI elements in _italic_.
Fix typos, add missing full-stops, add missing default values.
Co-authored-by: Rüdiger Herrmann <ruediger.herrmann@gmx.de>
013739c73d