llist.h comments are confusing about when locking is needed versus when it
isn't. Clarify these comments by being more descriptive about why locking is
needed for llist_del_first.

Cc: Ingo Molnar <mingo@kernel.org>
Cc: Will Deacon <will.deacon@arm.com>
Cc: Paul McKenney <paulmck@linux.vnet.ibm.com>
Acked-by: NHuang Ying <ying.huang@intel.com>
Acked-by: NMathieu Desnoyers <mathieu.desnoyers@efficios.com>
Signed-off-by: NJoel Fernandes <joelaf@google.com>
Signed-off-by: NPaul E. McKenney <paulmck@linux.vnet.ibm.com>
Reviewed-by: NJosh Triplett <josh@joshtriplett.org>