ada: Documentation for generic type inference

ada: Documentation for generic type inference

...plus minor improvements to existing documentation.

gcc/ada/

	* doc/gnat_rm/gnat_language_extensions.rst: I assume "extended set
	of extensions" was a typo for "experimental set of extensions",
	because "extended extensions" is repetitive and redundant. "in
	addition" clarifies that the one subsumes the other. Add a
	reminder at the start of each subsection about what switch/pragma
	enables what extensions. Add new section about "Inference of
	Dependent Types in Generic Instantiations".
	* gnat_rm.texi: Regenerate.