[LangRef] Fix condition for when a loop is considered parallel.

[Meinersbur]

Summary

The wording before this patch applies to llvm.mem.parallel_loop_access, not access groups.

[hfinkel]

LGTM

[mppf]

Thanks for fixing it!

llvm/docs/LangRef.rst

 ``g2`` (which might be identical). If a loop contains both access groups
 in its ``llvm.loop.parallel_accesses`` metadata, then the compiler can
 assume that there is no dependency between ``m1`` and ``m2`` carried by
 this loop. Instructions that belong to multiple access groups are
 considered having this property if at least one of the access groups
 matches the ``llvm.loop.parallel_accesses`` list.
 
 If all memory-accessing instructions in a loop have
-``llvm.loop.parallel_accesses`` metadata that refers to that loop, then the
+``llvm.access.group`` metadata that each refer to one of the access
+groups of a loop's ``llvm.loop.parallel_accesses`` metadata, then the
 loop has no loop carried memory dependences and is considered to be a
 parallel loop.
 
 Note that if not all memory access instructions belong to an access
 group referred to by ``llvm.loop.parallel_accesses``, then the loop must
 not be considered trivially parallel. Additional
 memory dependence analysis is required to make that determination. As a fail
 safe mechanism, this causes loops that were originally parallel to be considered