mdocml: 1) Remove documentation of the groff-1.15 compatibility quirk of

mdocml: 1) Remove documentation of the groff-1.15 compatibility quirk of

[schwarze]

Log Message:
-----------
1) Remove documentation of the groff-1.15 compatibility quirk
of suppressing spacing before a third .Xr argument
because that quirk was removed in mdoc_macro.c rev. 1.113.

2) Mark the "section" argument to .Xr as (syntactically) optional,
but still do not encourage omitting it.
The missing .Op was reported by espie@.

Wording tweaked by and ok jmc@, ok millert@.

Modified Files:
--------------
    mdocml:
        mdoc.7

Revision Data
-------------
Index: mdoc.7
===================================================================
RCS file: /usr/vhosts/mdocml.bsd.lv/cvs/mdocml/mdoc.7,v
retrieving revision 1.215
retrieving revision 1.216
diff -Lmdoc.7 -Lmdoc.7 -u -p -r1.215 -r1.216
--- mdoc.7
+++ mdoc.7
@@ -2532,20 +2532,14 @@ Link to another manual
 .Pq Qq cross-reference .
 Its syntax is as follows:
 .Pp
-.D1 Pf \. Sx \&Xr Ar name section
+.D1 Pf \. Sx \&Xr Ar name Op section
 .Pp
-The
+Cross reference the
 .Ar name
 and
 .Ar section
-are the name and section of the linked manual.
-If
-.Ar section
-is followed by non-punctuation, an
-.Sx \&Ns
-is inserted into the token stream.
-This behaviour is for compatibility with
-GNU troff.
+number of another man page;
+omitting the section number is rarely useful.
 .Pp
 Examples:
 .Dl \&.Xr mandoc 1
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