[librsvg] A few docs for SpecifiedValue

[Federico Mena Quintero <federico src gnome org>]

commit ecb6791d963c15a672598d0e481d1502e6216288
Author: Federico Mena Quintero <federico gnome org>
Date:   Mon May 21 20:01:06 2018 -0500

    A few docs for SpecifiedValue

 rsvg_internals/src/state.rs | 10 ++++++++++
 1 file changed, 10 insertions(+)
---
diff --git a/rsvg_internals/src/state.rs b/rsvg_internals/src/state.rs
index 64b230d6..0b72221c 100644
--- a/rsvg_internals/src/state.rs
+++ b/rsvg_internals/src/state.rs
@@ -24,6 +24,16 @@ use property_macros::Property;
 use unitinterval::UnitInterval;
 use util::{utf8_cstr, utf8_cstr_opt};
 
+/// Representation of a single CSS property value.
+///
+/// `Unspecified` is the `Default`; it means that the corresponding property is not present.
+///
+/// `Inherit` means that the property is explicitly set to inherit
+/// from the parent element.  This is useful for properties which the
+/// SVG or CSS specs mandate that should not be inherited by default.
+///
+/// `Specified` is a value given by the SVG or CSS stylesheet.  This will later be
+/// resolved into part of a `ComputedValues` struct.
 pub enum SpecifiedValue<T>
 where
     T: Property + Clone + Default,