This commit updates the section on LKMM limitations to no longer say
that SRCU is not modeled, but instead describe how LKMM's modeling of
SRCU departs from the Linux-kernel implementation.

TL;DR:  There is no known valid use case that cares about the Linux
kernel's ability to have partially overlapping SRCU read-side critical
sections.
Signed-off-by: Paul E. McKenney <paulmck@linux.ibm.com>
Acked-by: Andrea Parri <andrea.parri@amarulasolutions.com>