Improve the docs of GtkWidget and GtkGrid
I wanted to know how to use the GtkGrid as a GtkBuildable but I didn't find an answer in the docs of GtkGrid. However, there is an example in the documentation of GtkWidget but I didn't look for it there. This is why I added an example to GtkGrid.
By looking more closely to the docs of GtkWidget, I discovert two more minor typos.