graphics/cairo: make documentation available in devhelp

GNOME devhelp looks for documentation in particular places,
and cairo was putting it somewhere subtly different.
Switch DOCSDIR (since it was already being used in the
Makefile and pkg-plist, this is the smallest change)
to point to where it should. This approach is copied
from devel/libsoup/ and was suggested by Sean Champ.

PR:		263852 263785

graphics/cairo/Makefile

@@ -1,6 +1,6 @@
 PORTNAME=	cairo
 DISTVERSION=	1.17.4
-PORTREVISION=	1
+PORTREVISION=	2
 PORTEPOCH=	3
 CATEGORIES=	graphics
 MASTER_SITES=	https://cairographics.org/releases/ \
@@ -45,6 +45,7 @@ OPTIONS_SUB=	yes
 
 DOCS_BUILD_DEPENDS=	gtkdocize:textproc/gtk-doc
 DOCS_CONFIGURE_ON=	--with-html-dir=${DOCSDIR}
+DOCSDIR=		${PREFIX}/share/gtk-doc/html
 
 X11_USES=	xorg
 X11_USE=	xorg=x11,xext,xorgproto,xrender