ada: Adjust comments and doc about the new use of restriction No_Streams

ada: Adjust comments and doc about the new use of restriction No_Streams

Extend code comment; move recently added documentation from pragma
No_Tagged_Streams to restriction No_Streams.

gcc/ada/

	* doc/gnat_rm/implementation_defined_pragmas.rst
	(No_Tagged_Streams): Move documentation.
	* doc/gnat_rm/standard_and_implementation_defined_restrictions.rst
	(No_Streams): Likewise.
	* exp_disp.adb (Make_DT): Extend comment.
	* gnat_rm.texi: Regenerate.
	* gnat_ugn.texi: Regenerate.