[gtk/matthiasc/expression] Add some more expression docs

[Matthias Clasen <matthiasc src gnome org>]

commit 177264fdfc7095e845b35498747ea99e4eea5478
Author: Matthias Clasen <mclasen redhat com>
Date:   Fri Nov 29 11:36:14 2019 -0500

    Add some more expression docs

 gtk/gtkexpression.c | 38 ++++++++++++++++++++++++++++++++++++++
 1 file changed, 38 insertions(+)
---
diff --git a/gtk/gtkexpression.c b/gtk/gtkexpression.c
index 5cd0e26326..79deddcf18 100644
--- a/gtk/gtkexpression.c
+++ b/gtk/gtkexpression.c
@@ -45,6 +45,44 @@
  *
  * Expressions can be used to automatically update the propery of an object, similar
  * to GObject's #GBinding mechanism. Use gtk_expression_bind() for this.
+ *
+ * #GtkExpression in ui files
+ *
+ * GtkBuilder has support for creating expressions. The syntax here can be used where
+ * a #GtkExpression object is needed, or in a <binding> tag to bind a property to
+ * an expression.
+ *
+ * To create an property expression, use the <lookup> element. It can have a `type`
+ * attribute to specify the object type, and a `name` attribute to specify the property
+ * to look up. The content of <lookup> can either be an element specfiying the expression
+ * to use the object, or a string that specifies the name of the object to use.
+ * 
+ * Example:
+ * |[
+ *   <lookup name='search'>string_filter</lookup>
+ * |]
+ *
+ * To create a constant expression, use the <constant> element. If the type attribute
+ * is specified, the element content is interpreted as a value of that type. Otherwise,
+ * it is assumed to be an object.
+ *
+ * Example:
+ * |[
+ *   <constant>string_filter</constant>
+ *   <constant type='gchararray'>Hello, world</constant>
+ * ]|
+ *
+ * To create a closure expression, use the <closure> element. The `type` and `function`
+ * attributes specify what function to use for the closure, the content of the element
+ * contains the expressions for the parameters.
+ * 
+ * Example:
+ * |[
+ *   <closure type='gchararray' function='combine_args_somehow'>
+ *     <constant type='gchararray'>File size:</constant>
+ *     <lookup type='gint64' name='size'>myfile</lookup>
+ *   </closure>
+ * ]|
  */
 typedef struct _GtkExpressionClass GtkExpressionClass;