NEWS: quibbles from Tomi

See id:m2vbrsgi7n.fsf@guru.guru-group.fi for details
NEWS: Improve and correct "Fix for phrase indexing" entry

This improves the description of the fix, fixes some typos, and
changes "(re)-indexed" to "indexed" because we have no particular
notion of "re-indexing" a message.
version: bump to 0.18.1

Also add precis of NEWS to debian changelog
Merge tag '0.18.1'

notmuch 0.18.1 release