Clearify the meaning of function in widget reference

[GoogleCodeExporter]

The term "function" which is used in widget reference of at least button and 
entry could be clearer by pointing out to what kind of function it refers (a 
gtkdialog function or a shell function or something else). For the former case 
I'd suggest to add a link to a definition of gtkdialog function, for the latter 
to add the word shell (example link 
http://code.google.com/p/gtkdialog/wiki/button).

Original issue reported on code.google.com by [email protected] on 20 Apr 2014 at 3:35