[librsvg] Document DefsLookupErrorKind

[Federico Mena Quintero <federico src gnome org>]

commit c30e8b938f236e939ba5d5989074915d54dbaf1b
Author: Federico Mena Quintero <federico gnome org>
Date:   Mon May 4 18:38:42 2020 -0500

    Document DefsLookupErrorKind

 rsvg_internals/src/error.rs | 15 +++++++++++++++
 1 file changed, 15 insertions(+)
---
diff --git a/rsvg_internals/src/error.rs b/rsvg_internals/src/error.rs
index 58405dda..cf05f35d 100644
--- a/rsvg_internals/src/error.rs
+++ b/rsvg_internals/src/error.rs
@@ -94,10 +94,25 @@ impl fmt::Display for ElementError {
     }
 }
 
+/// Errors returned when looking up a resource by URL reference.
 #[derive(Debug, Clone, PartialEq)]
 pub enum DefsLookupErrorKind {
+    /// Error when parsing an [`Href`].
+    ///
+    /// [`Href`]: allowed_url/enum.Href.html
     HrefError(HrefError),
+
+    /// Used when the public API tries to look up an external URL, which is not allowed.
+    ///
+    /// This catches the case where a public API wants to be misused to access an external
+    /// resource.  For example, `SvgHandle.has_sub("https://evil.com/phone_home#element_id";) will
+    /// fail with this error.
     CannotLookupExternalReferences,
+
+    /// For internal use only.
+    ///
+    // FIXME: this is returned internally from Handle.lookup_node(), and gets translated
+    // to Ok(false).  Don't expose this internal code in the public API.
     NotFound,
 }