Fix some quoting glitches in doc strings

lisp/mh-e/mh-identity.el

@@ -282,9 +282,9 @@ If VALUE is nil, use `mh-extract-from-attribution-verb'."
 
 (defun mh-identity-handler-default (field action top &optional value)
   "Process header FIELD.
-The ACTION is one of 'remove or 'add. If TOP is non-nil, add the
+The ACTION is one of `remove' or `add'. If TOP is non-nil, add the
 field and its VALUE at the top of the header, else add it at the
-bottom of the header. If action is 'add, the VALUE is added."
+bottom of the header. If action is `add', the VALUE is added."
   (let ((field-colon (if (string-match "^.*:$" field)
                          field
                        (concat field ":"))))

lisp/mh-e/mh-letter.el

@@ -952,7 +952,7 @@ callable from a mouse button."
 (defun mh-insert-prefix-string (mh-ins-string)
   "Insert prefix string before each line in buffer.
 The inserted letter is cited using `sc-cite-original' if
-`mh-yank-behavior' is one of 'supercite or 'autosupercite.
+`mh-yank-behavior' is one of `supercite' or `autosupercite'.
 Otherwise, simply insert MH-INS-STRING before each line."
   (goto-char (point-min))
   (cond ((or (eq mh-yank-behavior 'supercite)

lisp/mh-e/mh-search.el

@@ -1329,7 +1329,7 @@ is used to search."
   "Read the next result.
 Parse it and return the message folder, message index and the
 match. If no other matches left then return nil. If the current
-record is invalid return 'error."
+record is invalid return `error'."
   (prog1
       (cl-block nil
         (when (eobp)