Use double sentence spacing and wrap lines at 'textwidth'. Code
examples and tables were not wrapped unless this had already been done
locally.
closes: vim/vim#18453c58f91c035
Fix incorrect docs in :h ModeChanged.
Cherry-pick :h bufnr() changes from patch 8.1.2080.
Co-authored-by: Doug Kearns <dougkearns@gmail.com>