man-pages.7: Expand the guidance on formatting code snippets

Signed-off-by: Michael Kerrisk <mtk.manpages@gmail.com>

man7/man-pages.7

@@ -635,7 +635,32 @@ When structure definitions, shell session logs, and so on are included
 in running text, indent them by 4 spaces (i.e., a block enclosed by
 .I ".in\ +4n"
 and
-.IR ".in" ).
+.IR ".in" ),
+format them using the
+.I .EX
+and
+.I EE
+macros, and surround them with suitable paragraph markers (either
+.I .PP
+or
+.IR .IP ).
+For example:
+.PP
+.in +4n
+.EX
+    .PP
+    .in +4n
+    .EX
+    int
+    main(int argc, char *argv[])
+    {
+        return 0;
+    }
+    .EE
+    .in
+    .PP
+.EE
+.in
 .SS Preferred terms
 The following table lists some preferred terms to use in man pages,
 mainly to ensure consistency across pages.