list.h: Update comment to explicitly note circular lists

The students in the Operating System Lecture Section at the
American University of Sharjah were confused by the header comment
in include/linux/list.h, which says "Simple doubly linked list
implementation".  This comment means "simple" as in "not complex",
but "simple" is often used in this context to mean "not circular".
This commit therefore avoids this ambiguity by explicitly calling out
"circular".

Signed-off-by: Asif Rasheed <b00073877@aus.edu>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>