emacs: update defcustom for notmuch-search-result-format.

It seems redundant to have the previous example, since the default
value is always show by describe variable.

Enforce more restrictions on the keys in the alist, since arbitrary
strings don't work as field names.

Document that functions can be used in lieu of field names.

emacs/notmuch.el

@@ -90,11 +90,11 @@
     ("tags" . "(%s)"))
   "Search result formatting.
 
-Supported fields are: date, count, authors, subject, tags.
-For example:
-    (setq notmuch-search-result-format
-          \\='((\"authors\" . \"%-40s\")
-            (\"subject\" . \"%s\")))
+List of pairs of (field . format-string).  Supported field
+strings are: \"date\", \"count\", \"authors\", \"subject\",
+\"tags\".  It is also supported to pass a function in place of a
+field name. In this case the function is passed the thread
+object (plist) and format string.
 
 Line breaks are permitted in format strings (though this is
 currently experimental).  Note that a line break at the end of an
@@ -102,7 +102,16 @@ currently experimental).  Note that a line break at the end of an
 place it instead at the beginning of the following field.  To
 enter a line break when setting this variable with setq, use \\n.
 To enter a line break in customize, press \\[quoted-insert] C-j."
-  :type '(alist :key-type string :value-type string)
+  :type '(alist
+	  :key-type
+	  (choice
+	   (const :tag "Date" "date")
+	   (const :tag "Count" "count")
+	   (const :tag "Authors" "authors")
+	   (const :tag "Subject" "subject")
+	   (const :tag "Tags" "tags")
+	   function)
+	  :value-type (string :tag "Format"))
   :group 'notmuch-search)
 
 ;; The name of this variable `notmuch-init-file' is consistent with the