Don't mention very old Emacs versions in docs

* doc/misc/mh-e.texi (Conventions):
* doc/misc/reftex.texi (Problems and Work-Arounds):
* doc/misc/viper.texi (Loading Viper): Delete references to
very old versions of Emacs.

doc/misc/mh-e.texi

@@ -333,8 +333,7 @@ Press the @key{TAB} key.
 Press the @key{DELETE} key.
 @c -------------------------
 @item @key{BS}
-Press the @key{BACKSPACE} key@footnote{If you are using Version 20 or
-earlier of Emacs, you will need to use the @key{DEL} key.}.
+Press the @key{BACKSPACE} key.
 @end table
 
 @cindex Emacs, prefix argument

doc/misc/reftex.texi

@@ -3539,18 +3539,6 @@ as a label of type @code{?p}.  Argument count for this macro starts only
 after the @samp{@{step+@}}, also when specifying how to get
 context.
 
-@item
-@b{Viper mode}@*
-@cindex Viper mode
-@cindex Key bindings, problems with Viper mode
-@findex viper-harness-minor-mode
-With @i{Viper} mode prior to Vipers version 3.01, you need to protect
-@RefTeX{}'s keymaps with
-
-@lisp
-(viper-harness-minor-mode "reftex")
-@end lisp
-
 @end itemize
 
 @page

doc/misc/viper.texi

@@ -325,9 +325,9 @@ lines (in the given order!):
 @noindent
 in your @file{~/.emacs} file.  The @file{.emacs} file is placed in your
 home directory and it is be executed every time you invoke Emacs.  This is
-the place where all general Emacs customization takes place.  Beginning with
-version 20.0, Emacsen have an interactive interface, which simplifies the
-job of customization significantly.
+the place where all general Emacs customization takes place.  Emacs
+has an interactive interface (@kbd{M-x customize}), which simplifies
+the job of customization significantly.
 
 Viper also uses the file @file{~/.emacs.d/viper} for Viper-specific customization.
 The location of Viper customization file can be changed by setting the