Some recent changes to the composer layout need to be reflected in the user
manual.
Also added an explanation of what the Cc, Bcc, and Reply-To fields do.
Fixed some typos and improved wording elsewhere on the page.
Update pages to be consistent with how the app currently works. Mention
undo for both archiving and sending/saving/dicarding composers. Update
compser page to mention features added over the last few releases.
