cogl-path: Document how a shape is filled

This adds some documentation to cogl_path_fill() describing the fill
rule Cogl uses.

doc/reference/cogl/Makefile.am

@@ -90,7 +90,8 @@ EXTRA_HFILES=
 
 # Images to copy into HTML directory.
 # e.g. HTML_IMAGES=$(top_srcdir)/gtk/stock-icons/stock_about_24.png
-HTML_IMAGES=
+HTML_IMAGES = \
+	fill-rule.png
 
 # Extra SGML files that are included by $(DOC_MAIN_SGML_FILE).
 # e.g. content_files=running.sgml building.sgml changes-2.0.sgml
@@ -117,3 +118,6 @@ include $(top_srcdir)/gtk-doc.make
 
 # Other files to distribute
 # e.g. EXTRA_DIST += version.xml.in
+
+EXTRA_DIST += \
+	fill-rule.png