Misc documentation fixes

• Fix markup in 9.4 release notes
• Document -ddump-cs-trace
• Mention that ImpredicativeTypes is really supported only since 9.2
• Remove "There are some restrictions on the use of unboxed tuples". This used to be a list, but all those restrictions were removed.
• Mark -fimplicit-import-qualified as documented
• Remove "The :main and :run command" - duplicated verbatim in options
• Avoid calling "main" a function (cf. #7816 (closed))
• Update System.getArgs: the old location was before hierarchical modules
• Note that multiplicity multiplication is not supported (#20319 (closed))
• Use https links (separate commit for ease of review)