docs: brush up the TODO list

TODO

-TODO file (? means the feature may be implemented, but not definitely)
-------------------------------------------------------------------------
+A list of desired features
+--------------------------
 
-For the future (no targeted version, catch-all)
+Vague musings:
 - FriBidi support?
-- Port to DJGPP?
 - Make matching bracket searches sophisticated enough to skip over
   brackets inside comments?
 - Allow indentation of marked text by spaces as well as tabs?
@@ -12,26 +11,27 @@ For the future (no targeted version, catch-all)
 - Allow conversion between different character sets.  Maybe use glib's
   iconv() if the system's iconv() is inadequate, since we already use
   glib's vsnprintf() if the system lacks vsnprintf()?
-- Allow color syntaxes to apply to more than just color, so that we can
-  e.g. specify a different alternate spell checker depending on which
-  file type we have open.
 - Allow setting marks (saved positions, not to be confused with the mark
   set via Ctrl-^) at various lines and/or columns in the buffer, and
-  allow movement between them with a single keystroke?  (We're running
-  out of keystrokes, though.)
-- Allow searching for and replacing newlines.
-
-For version 2.6:
-- New regression framework built on expect.
-- Allow text searches in the help viewer.
+  allow movement between them with a single keystroke?
+- Allow searching for and replacing newlines?
+  See also https://savannah.gnu.org/bugs/?47053.
+- New regression framework built on expect?
 - Add the ability to move to different lines of the screen with a single
   keystroke, e.g. M-` (M-~) to go to the top line, M-& (M-7) to go to
-  the center line, and M-' (M-") to go to the last line.
-- Detect when we're pasting text, so that we can handle it differently
-  (i.e. faster than currently with screen redraws).
+  the center line, and M-' (M-") to go to the last line?
+
+Somewhat urgent:
+- Allow text searches in the help viewer.
+  See https://savannah.gnu.org/bugs/?28994.
+- Detect when text is being pasted, so that we can handle it faster.
+  See https://savannah.gnu.org/bugs/?40060.
+- Allow color syntaxes to apply to more than just color, so that we can
+  e.g. specify a different alternate spell checker depending on which
+  file type we have open.
 - Allow piping marked text to an external command and replacing it with
-  the command's output (as the internal spell checker already does with
-  the "spell" command)?
+  the command's output -- that is: generalize the spell-check flow.
+  See also https://savannah.gnu.org/bugs/?28993.
 
 For version 2.4:
 - Handle window resizes better.  After we've resized, we should stay