[Ada] Adjust description of Pure_Function pragma

The current wording can be read as implying that the result of a call
to a pure function does not depend on the context, which is incorrect.
The pragma only guarantees the absence of side effects of such a call.

gcc/ada/

	* doc/gnat_rm/implementation_defined_pragmas.rst (Pure_Function):
	Fix ambiguous wording about context dependence.
	* gnat_rm.texi: Regenerate.
2 files changed