[gtk+] Document rem as CSS unit

[Matthias Clasen <matthiasc src gnome org>]

commit f60e144038a50c48ebbb928f4bf1bd8208d48f86
Author: Matthias Clasen <mclasen redhat com>
Date:   Fri Feb 5 06:53:11 2016 +0100

    Document rem as CSS unit

 docs/reference/gtk/css-properties.xml |    7 ++++++-
 1 files changed, 6 insertions(+), 1 deletions(-)
---
diff --git a/docs/reference/gtk/css-properties.xml b/docs/reference/gtk/css-properties.xml
index 09fcc0b..3b80b4a 100644
--- a/docs/reference/gtk/css-properties.xml
+++ b/docs/reference/gtk/css-properties.xml
@@ -53,7 +53,7 @@ We use <literallayout> for syntax productions, and each line is put in a <code>
       The following basic datatypes are used throughout:
     </para>
 
-<literallayout><code>〈length〉 = 〈number〉 [ px | pt | em | ex | pc | in | cm | mm ]</code>
+<literallayout><code>〈length〉 = 〈number〉 [ px | pt | em | ex |rem | pc | in | cm | mm ]</code>
 <code>〈percentage〉 = 〈number〉 %</code>
 <code>〈angle〉 = 〈number〉 [ deg | grad | turn ]</code>
 <code>〈time〉 = 〈number〉 [ s | ms ]</code>
@@ -66,6 +66,11 @@ We use <literallayout> for syntax productions, and each line is put in a <code>
     </para>
 
     <para>
+      The rem unit is resolved using the initial font size value, which is
+      not quite the same as the CSS definition of rem.
+    </para>
+
+    <para>
       A common pattern among shorthand properties (called 'four sides') is one
       where one to four values can be specified, to determine a value for each
       side of an area. In this case, the specified values are interpreted as