Don't fill keywords after Emacs Lisp docstring

This approach does mean that keywords that have spaces before them
inside of docstrings aren't filled, but I think this is should be fine
until Bug#28937 is fixed.

* lisp/emacs-lisp/lisp-mode.el (lisp-fill-paragraph): Add a colon to
paragraph-start unconditionally, but require that it follows at least
one space.  (Bug#24622)
* test/lisp/emacs-lisp/lisp-tests.el: New tests for Bug#24622 and
Bug#7751.

lisp/emacs-lisp/lisp-mode.el

@@ -1267,7 +1267,8 @@ and initial semicolons."
       ;; case).  The `;' and `:' stop the paragraph being filled at following
       ;; comment lines and at keywords (e.g., in `defcustom').  Left parens are
       ;; escaped to keep font-locking, filling, & paren matching in the source
-      ;; file happy.
+      ;; file happy.  The `:' must be preceded by whitespace so that keywords
+      ;; inside of the docstring don't start new paragraphs (Bug#7751).
       ;;
       ;; `paragraph-separate': A clever regexp distinguishes the first line of
       ;; a docstring and identifies it as a paragraph separator, so that it
@@ -1280,13 +1281,7 @@ and initial semicolons."
       ;; `emacs-lisp-docstring-fill-column' if that value is an integer.
       (let ((paragraph-start
              (concat paragraph-start
-                     (format "\\|\\s-*\\([(;%s\"]\\|`(\\|#'(\\)"
-                             ;; If we're inside a string (like the doc
-                             ;; string), don't consider a colon to be
-                             ;; a paragraph-start character.
-                             (if (nth 3 (syntax-ppss))
-                                 ""
-                               ":"))))
+                     "\\|\\s-*\\([(;\"]\\|\\s-:\\|`(\\|#'(\\)"))
 	    (paragraph-separate
 	     (concat paragraph-separate "\\|\\s-*\".*[,\\.]$"))
             (fill-column (if (and (integerp emacs-lisp-docstring-fill-column)

test/lisp/emacs-lisp/lisp-tests.el

@@ -589,5 +589,36 @@ region."
     (should (= (point) before))
     (should (= (mark) after))))
 
+(ert-deftest lisp-fill-paragraph-colon ()
+  "Keywords below Emacs Lisp docstrings should not be filled (Bug#24622).
+Keywords inside docstrings should be filled (Bug#7751)."
+  (elisp-tests-with-temp-buffer
+      "
+\(defcustom custom value
+  \"First\n
+Second\n
+=!inside=Third line\"
+  =!keywords=:type 'sexp
+  :version \"26.1\"
+  :group 'lisp-tests)"
+    (goto-char inside)
+    (fill-paragraph)
+    (goto-char keywords)
+    (beginning-of-line)
+    (should (looking-at "  :type 'sexp\n  :version \"26.1\"\n  :")))
+  (elisp-tests-with-temp-buffer
+      "
+\(defun foo ()
+  \"Summary.
+=!inside=Testing keywords: :one :two :three\"
+  (body))" ; FIXME: Remove parens around body to test Bug#28937 once it's fixed
+    (goto-char inside)
+    (let ((emacs-lisp-docstring-fill-column 30))
+      (fill-paragraph))
+    (forward-line)
+    (should (looking-at ":three"))
+    (end-of-line)
+    (should-not (eq (preceding-char) ?\)))))
+
 (provide 'lisp-tests)
 ;;; lisp-tests.el ends here