Improve documentation of some Help commands

* doc/emacs/help.texi (Help Mode): Fix mistake in key sequences.
Mention the 'l' and 'r' keys.
(Misc Help): Mention that 'C-h m' describes key bindings.
(Bug#24890)

doc/emacs/help.texi

@@ -411,8 +411,10 @@ Follow a hyperlink that you click on.
 Show all documentation about the symbol at point
 (@code{help-follow-symbol}).
 @item C-c C-f
+@itemx r
 Go forward to the next help topic (@code{help-go-forward}).
 @item C-c C-b
+@itemx l
 Go back to the previous help topic (@code{help-go-back}).
 @end table
 
@@ -422,15 +424,18 @@ Go back to the previous help topic (@code{help-go-back}).
 @findex help-go-forward
 @kindex RET @r{(Help mode)}
 @kindex C-c C-b @r{(Help mode)}
+@kindex l @r{(Help mode)}
 @kindex C-c C-f @r{(Help mode)}
+@kindex r @r{(Help mode)}
   When a function name, variable name, or face name (@pxref{Faces})
 appears in the documentation in the help buffer, it is normally an
 underlined @dfn{hyperlink}.  To view the associated documentation,
 move point there and type @key{RET} (@code{help-follow}), or click on
 the hyperlink with @kbd{mouse-1} or @kbd{mouse-2}.  Doing so replaces
 the contents of the help buffer; to retrace your steps, type @kbd{C-c
-C-b} (@code{help-go-back}).  While retracing your steps, you can go
-forward by using @kbd{C-c C-b} (@code{help-go-forward}).
+C-b} or @kbd{l} (@code{help-go-back}).  While retracing your steps,
+you can go forward by using @kbd{C-c C-f} or @kbd{r}
+(@code{help-go-forward}).
 
 @cindex URL, viewing in help
 @cindex help, viewing web pages
@@ -556,7 +561,8 @@ commands that you are not familiar with, you can use @kbd{C-h k} or
   Each Emacs major mode typically redefines a few keys and makes other
 changes in how editing works.  @kbd{C-h m} (@code{describe-mode})
 displays documentation on the current major mode, which normally
-describes the commands and features that are changed in this mode.
+describes the commands and features that are changed in this mode, and
+also its key bindings.
 
 @kindex C-h b
 @findex describe-bindings