added missing combobox ctor param doc

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/branches/WX_2_4_BRANCH@17934 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775

docs/latex/wx/combobox.tex

@@ -67,6 +67,8 @@ Constructor, creating and showing a combobox.
 
 \docparam{id}{Window identifier. A value of -1 indicates a default value.}
 
+\docparam{value}{Initial selection string. An empty string indicates no selection.}
+
 \docparam{pos}{Window position.}
 
 \docparam{size}{Window size. If the default size (-1, -1) is specified then the window is sized