Clarify when C-? is available as Help.

Explain that normal-erase-is-backspace-mode toggles between the two modes.
Describe the case where BACKSPACE has been remapped to DELETE
in the window system.

man/trouble.texi

@@ -169,16 +169,20 @@ character.  Emacs ought to be treating @key{BS} as @key{DEL}, but it
 isn't.
 
   In all of those cases, the immediate remedy is the same: use the
-command @kbd{M-x normal-erase-is-backspace-mode}.  That should make
-the proper key work as @key{DEL}.  On a text-only terminal, if you do
-want to ask for help, use @key{F1} or @kbd{C-?}.
+command @kbd{M-x normal-erase-is-backspace-mode}.  This toggles
+between the two modes that Emacs supports for handling @key{DEL}, so
+if Emacs starts in the wrong mode, it should switch to the right mode.
+On a text-only terminal, if you want to ask for help when @key{BS} is
+treated as @key{DEL}, use @key{F1}; @kbd{C-?} may also work, if it
+sends character code 127.
 
 @findex normal-erase-is-backspace-mode
   To fix the problem automatically for every Emacs session, you can
 put one of the following lines into your @file{.emacs} file
 (@pxref{Init File}).  For the first case above, where @key{DELETE}
 deletes forwards instead of backwards, use this line to make
-@key{DELETE} act as @key{DEL}:
+@key{DELETE} act as @key{DEL} (resulting in behavior compatible
+with Emacs 20 and previous versions):
 
 @lisp
 (normal-erase-is-backspace-mode 0)
@@ -199,6 +203,15 @@ customize the variable @code{normal-erase-is-backspace}: the value
 @key{DEL}, and @code{nil} specifies the other mode.  @xref{Easy
 Customization}.
 
+  With a window system, it can also happen that the usual erasure key
+is labeled @key{BACKSPACE}, there is a @key{DELETE} key elsewhere, and
+both keys delete forward.  This probably means that someone has
+redefined your @key{BACKSPACE} key as a @key{DELETE} key.  With X,
+this is typically done with a command to the @code{xmodmap} program
+when you start the server or log in.  The most likely motive for this
+customization was to support old versions of Emacs, so we recommend
+you simply remove it now.
+
 @node Stuck Recursive
 @subsection Recursive Editing Levels