style.9: Codify tolerance for eliding blank lines

Consensus seems to be that eliding blank lines for functions with no local
variables is acceptable.  Codify that explicitly in the style document.

Reported by:	jhb
Reviewed by:	delphij, imp, vangyzen (earlier version); rgrimes
With feedback from:	kib
Differential Revision:	https://reviews.freebsd.org/D20448

share/man/man9/style.9

@@ -26,7 +26,7 @@
 .\"	From: @(#)style	1.14 (Berkeley) 4/28/95
 .\" $FreeBSD$
 .\"
-.Dd May 28, 2019
+.Dd June 3, 2019
 .Dt STYLE 9
 .Os
 .Sh NAME
@@ -779,9 +779,20 @@ vaf(const char *fmt, ...)
 static void
 usage(void)
 {
-	/* Insert an empty line if the function has no local variables. */
+	/* Optional blank line goes here. */
 .Ed
 .Pp
+Optionally, insert a blank line at the beginning of functions with no local
+variables.
+Older versions of this
+.Nm
+document required the blank line convention, so it is widely used in existing
+code.
+.Pp
+Do not insert a blank line at the beginning of functions with local variables.
+Instead, these should have local variable declarations first, followed by one
+blank line, followed by the first statement.
+.Pp
 Use
 .Xr printf 3 ,
 not