Update documentation

Adds new entry to the NEWS file, and updates the search terms section
of the man page.  The search terms section needs to be updated again
once the new section in the documentation covering probablistic terms
has been committed.

NEWS

@@ -1,4 +1,14 @@
 Notmuch 0.20 (UNRELEASED)
+=========================
+
+Command-Line Interface
+----------------------
+
+There is a new `mimetype:` search prefix
+
+  The new `mimetype:` search prefix allows searching for the
+  content-type of attachments, which is now indexed. See the
+  `notmuch-search-terms` manual page for details.
 
 Documentation
 -------------

doc/man7/notmuch-search-terms.rst

@@ -40,6 +40,8 @@ indicate user-supplied values):
 
 -  attachment:<word>
 
+-  mimetype:<word>
+
 -  tag:<tag> (or is:<tag>)
 
 -  id:<message-id>
@@ -66,6 +68,10 @@ by including quotation marks around the phrase, immediately following
 The **attachment:** prefix can be used to search for specific filenames
 (or extensions) of attachments to email messages.
 
+The **mimetype:** prefix will be used to match text from the
+content-types of MIME parts within email messages (as specified by the
+sender).
+
 For **tag:** and **is:** valid tag values include **inbox** and
 **unread** by default for new messages added by **notmuch new** as well
 as any other tag values added manually with **notmuch tag**.