English user guide: place a line break between each sentence within p…

…aragraphs in the user guide. This will aide translators in future as then diffs will much more clearly show what sentences have changed. Note that the breaks only appear in the source, and not the HTML output. Paragraphs that act as descriptions for key commands have not been split as the processing to generate the key commands document only grabs the first line. This commit does not change any content in the user guide, it only changes formatting.

Fixes #855.