Fix a few compiler warnings. Fix crash with encrypted undo file.

runtime/doc/editing.txt

@@ -1376,6 +1376,9 @@ check if the encryption works as expected.  If you get one of these errors
 don't write the file encrypted!  You need to rebuild the Vim binary to fix
 this.
 
+*E831* This is an internal error, "cannot happen".  If you can reproduc it,
+please report to the developers.
+
 When reading a file that has been encrypted and the 'key' option is not empty,
 it will be used for decryption.  If the value is empty, you will be prompted
 to enter the key.  If you don't enter a key, or you enter the wrong key, the

runtime/doc/tags

@@ -4183,10 +4183,14 @@ E822	undo.txt	/*E822*
 E823	undo.txt	/*E823*
 E824	undo.txt	/*E824*
 E825	undo.txt	/*E825*
+E826	undo.txt	/*E826*
+E827	undo.txt	/*E827*
 E828	undo.txt	/*E828*
 E829	undo.txt	/*E829*
 E83	message.txt	/*E83*
 E830	undo.txt	/*E830*
+E831	editing.txt	/*E831*
+E832	undo.txt	/*E832*
 E84	windows.txt	/*E84*
 E85	options.txt	/*E85*
 E86	windows.txt	/*E86*

runtime/doc/todo.txt

@@ -30,6 +30,8 @@ be worked on, but only if you sponsor Vim development.  See |sponsor|.
 							*known-bugs*
 -------------------- Known bugs and current work -----------------------
 
+gtk_selection_clear_targets not available in GTK1 (Patrick Texier)
+
 Cursor positioning wrong with 0x200e character. (John Becket, 2010 May 6)
 
 E315 when trying to change a file in FileChangedRO autocommand event.

runtime/doc/undo.txt

@@ -285,6 +285,13 @@ Reading an existing undo file may fail for several reasons:
 	the undo file cannot be used, it would corrupt the text.  This also
 	happens when 'encoding' differs from when the undo file was written.
 *E825*  The undo file does not contain valid contents and cannot be used.
+*E826*  The undo file is encrypted but decryption failed.
+*E827*  The undo file is encrypted but this version of Vim does not support
+	encryption.  Open the file with another Vim.
+*E832*  The undo file is encrypted but 'key' is not set, the text file is not
+	encrypted.  This would happen if the text file was written by Vim
+	encrypted at first, and later overwritten by not encrypted text.
+	You probably want to delete this undo file.
 "Not reading undo file, owner differs"
 	The undo file is owned by someone else than the owner of the text
 	file.  For safety the undo file is not used.