[Ada] Fix documentation of using attribute Loop_Entry in pragmas

[Ada] Fix documentation of using attribute Loop_Entry in pragmas

Attribute Loop_Entry was initially only allowed to appear in pragmas
Loop_Variant and Loop_Invariant. Then it was also allowed to appear in
pragmas Assert, Assert_And_Cut and Assume, but this change was not
reflected in the GNAT RM.

gcc/ada/

	* doc/gnat_rm/implementation_defined_attributes.rst
	(Loop_Entry): Mention pragmas Assert, Assert_And_Cut and Assume;
	refill.
	* gnat_rm.texi: Regenerate.