proposed doc fix: link to "auto-increment" from "setting registers"

[Marc Chantreux]

hello people,

`info groff Req nr` leads you to the "setting registers" section which
doesn't help when you try to understand `.nr H1 0 1` (from ms). it would
be nice to have at least a reference to this other section.

both my texi and my english are terrible but i wrote this patch as an
exemple of what would have helped me to get faster to the point:

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diff --git a/doc/groff.texi b/doc/groff.texi
index 317d94d4..7baa7c0f 100644
--- a/doc/groff.texi
+++ b/doc/groff.texi
@@ -5682,6 +5682,9 @@ set to zero.
 Set number register @var{ident} to @var{value}.  If @var{ident} doesn't
 exist, @code{gtroff} creates it.
 
+@DefreqList {nr, ident value step} is described in the
+@xref{Auto-increment} section.
+
 The argument to @code{\R} usually has to be enclosed in quotes.
 @xref{Escapes}, for details on parameter delimiting characters.
 
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Regards,
marc