Signal parameter names in introspection annotations aren't forced to be valid C identifiers because there is no corresponding C function in the source code.   

GdkWindow::from-embedder and GdkWindow::to-embedder use parameter names
  embedder-x
  embedder-y
  offscreen-x
  offscreen-y

Consequently, signal handler C function prototypes in reference documentation are not valid C, e.g.

  void user_function (GdkWindow *window,
                      gdouble    offscreen-x,
                      gdouble    offscreen-y,
                      gpointer   embedder-x,
                      gpointer   embedder-y,
                      gpointer   user_data) : Run Last

Discussion on #introspection has established that signal parameter names should be valid C identifiers:

<dieterv> pclayton: GTK-Doc has this to say about singal parameter names:
<dieterv> # allow alphanumerics, '_', '[' & ']' in param names
<dieterv> if ($params[$j] =~ m/^\s*(\w+)\s*(\**)\s*([\w\[\]]+)\s*$/) {
<dieterv> so yeah, looks like they should be valic C identifiers
<dieterv> ... it happens in GetSignals, http://git.gnome.org/browse/gtk-doc/tree/gtkdoc-mkdb.in#n3399