[Ada] Document Aggregate_Individually_Assign as a configuration pragma

[Ada] Document Aggregate_Individually_Assign as a configuration pragma

Pragma Aggregate_Individually_Assign was apparently forgotten to be
listed in the GNAT User's Guide as a configuration pragma.

gcc/ada/

	* doc/gnat_ugn/the_gnat_compilation_model.rst (Configuration
	Pragmas): Add Aggregate_Individually_Assign; otherwise the list
	is complete except for some obsoleted pragmas, which most likely
	are intentionally omitted.
	* gnat_ugn.texi: Regenerate.