fix sum moar typos [user-visible documentation in code]

fix sum moar typos [user-visible documentation in code]

Various typo fixes in documentation within the code that can be made
available to the user, (emacs function help strings, "notmuch help"
output, notmuch man page, etc.).

Signed-off-by: Pieter Praet <pieter@praet.org>
Edited-by: Carl Worth <cworth@cworth.org> Restricted to just
documentation and fixed fix of "comman" to "common" rather than
"command".