This explicitly documents the behavior for /include/-ending paths and also
updates other parts of the description.

Tested on x86_64-pc-linux-gnu, committed on trunk

gcc/ada/

	* doc/gnat_ugn/the_gnat_compilation_model.rst (Binding generation):
	Document specific behavior for /include/-ending paths and update.
	* gnat_ugn.texi: Regenerate.