[librsvg] Document the purpose of the IRI type

[Federico Mena Quintero <federico src gnome org>]

commit c03130728490ebfa8d9c4369e4f175de4e483fc0
Author: Federico Mena Quintero <federico gnome org>
Date:   Thu Nov 29 15:40:10 2018 -0600

    Document the purpose of the IRI type

 rsvg_internals/src/iri.rs | 6 ++++++
 1 file changed, 6 insertions(+)
---
diff --git a/rsvg_internals/src/iri.rs b/rsvg_internals/src/iri.rs
index 71ebbf4a..03347171 100644
--- a/rsvg_internals/src/iri.rs
+++ b/rsvg_internals/src/iri.rs
@@ -3,6 +3,12 @@ use cssparser::Parser;
 use parsers::Parse;
 use parsers::ParseError;
 
+/// Used where style properties take a funciri or "none"
+///
+/// This is not to be used for values which don't come from properties.
+/// For example, the `xlink:href` attribute in the `<image>` element
+/// does not take a funciri value (which looks like `url(...)`), but rather
+/// it takes a plain URL.  Use the `Href` type in that case.
 #[derive(Debug, Clone, PartialEq)]
 pub enum IRI {
     None,