docs: remove references to not being able to undo justifications

Any series of justifications (and other edits) is now undoable.

doc/nano.1

@@ -308,12 +308,6 @@ it unique.  In multibuffer mode, \fBnano\fP will write all the open
 buffers to their respective emergency files.
 
 .SH BUGS
-Justifications (\fB^J\fR)
-are not yet covered by the general undo system.  So after a justification
-that is not immediately undone, earlier edits
-cannot be undone any more.  The workaround is, of course, to exit without
-saving.
-.sp
 The recording and playback of keyboard macros works correctly only on a
 terminal emulator, not on a Linux console (VT), because the latter does
 not by default distinguish modified from unmodified arrow keys.

doc/nano.texi

@@ -575,12 +575,6 @@ is running.
 @node Limitations
 @section Limitations
 
-Justifications (@kbd{^J})
-are not yet covered by the general undo system.  So after a justification
-that is not immediately undone, earlier edits
-cannot be undone any more.  The workaround is, of course, to exit without
-saving.
-
 The recording and playback of keyboard macros works correctly only on a
 terminal emulator, not on a Linux console (VT), because the latter does
 not by default distinguish modified from unmodified arrow keys.