docs: complete the renaming of 'tempfile' to 'saveonexit'

This should have been part of commit d4ee6a2b from two months ago.

doc/nano.1

@@ -302,6 +302,7 @@ using the built-in corrector that calls \fBhunspell\fR or GNU \fBspell\fR.
 .TP
 .BR \-t ", " \-\-saveonexit
 Save a changed buffer without prompting (when exiting with \fB^X\fR).
+(The old form of the long option, \fB\-\-tempfile\fR, is deprecated.)
 .TP
 .BR \-u ", " \-\-unix
 Save a file by default in Unix format.  This overrides nano's

doc/nano.texi

@@ -426,6 +426,7 @@ to be installed.
 Save a changed buffer without prompting (when exiting with @kbd{^X}).
 This can be handy when @command{nano} is used as the composer of an
 email program.
+(The old form of the long option, @code{--tempfile}, is deprecated.)
 
 @item -u
 @item --unix
@@ -945,6 +946,10 @@ system either Backspace acts like Delete or Delete acts like Backspace.
 Do regular-expression searches by default.
 Regular expressions in @command{nano} are of the extended type (ERE).
 
+@item set saveonexit
+Save a changed buffer automatically on exit (@kbd{^X}); don't prompt.
+(The old form of this option, @code{set tempfile}, is deprecated.)
+
 @item set selectedcolor [bold,][italic,]@var{fgcolor},@var{bgcolor}
 Use this color combination for selected text.
 @xref{@code{set functioncolor}} for valid color names.
@@ -992,9 +997,6 @@ greater than 0.  The default value is @t{8}.
 @item set tabstospaces
 Convert typed tabs to spaces.
 
-@item set tempfile
-Save automatically on exit, don't prompt.
-
 @item set titlecolor [bold,][italic,]@var{fgcolor},@var{bgcolor}
 Use this color combination for the title bar.
 @xref{@code{set functioncolor}} for valid color names.

doc/nanorc.5

@@ -258,6 +258,7 @@ Regular expressions in \fBnano\fR are of the extended type (ERE).
 .TP
 .B set saveonexit
 Save a changed buffer automatically on exit (\fB^X\fR); don't prompt.
+(The old form of this option, \fBset tempfile\fR, is deprecated.)
 .TP
 .B set selectedcolor \fR[\fBbold,\fR][\fBitalic,\fR]\fIfgcolor\fB,\fIbgcolor\fR
 Specify the color combination to use for selected text.