[gtksourceview] docs: clarify PangoUnderline support in style schemes

[Sébastien Wilmet <swilmet src gnome org>]

commit 74294f04afc97a0464c994018c8fd41ce5fe798a
Author: Sébastien Wilmet <swilmet gnome org>
Date:   Wed May 13 12:09:01 2015 +0200

    docs: clarify PangoUnderline support in style schemes
    
    "Up until version 3.18" was not directly clear to me. Does it meant that
    3.18 also supported only true or false?

 docs/reference/style-reference.xml |    2 +-
 1 files changed, 1 insertions(+), 1 deletions(-)
---
diff --git a/docs/reference/style-reference.xml b/docs/reference/style-reference.xml
index 35c5b95..452e7ea 100644
--- a/docs/reference/style-reference.xml
+++ b/docs/reference/style-reference.xml
@@ -145,7 +145,7 @@ Background color.
 <term><code>underline</code></term>
 <listitem><para>
 Accepts the values supported by PangoUnderline ("none", "single",
-"double", "low", "error"). Up until version 3.18 it only
+"double", "low", "error"). GtkSourceView versions &lt;= 3.16 only
 supported "true" or "false" and such value are still accepted
 for backward compatibility.
 </para></listitem>