Fix documentation of Modus Themes

Fix documentation of Modus Themes

* doc/misc/modus-themes.texi (Install from the archives)
(No mixed fonts): Remove references to MELPA.
(How do the themes look like)
(Enable and load, Load automatically)
(Configure options prior to loading, Customisation Options)
(No mixed fonts, Command prompts, Mode line, Completion UIs)
(Fringes, Line highlighting, Matching parentheses, Diffs)
(Org mode blocks, Heading styles, Tweak colors (DIY))
(Org user faces (DIY), Supported packages)
(Will NOT be supported, Note for ERC escaped color sequences)
(Note on shr colors, Note for Helm grep)
(Note on vc-annotate-background-mode, Sources of the themes): Fix
spelling, wording, and markup.