doc: Ensure whatisRCU.txt actually says what RCU is

It came to my attention that the file "whatisRCU.txt" does not
manage to actually ever spell out what is RCU.

This might not be an issue for a lot of people, but we have to
assume the consumers of these documents are starting from ground
zero; otherwise they'd not be reading the docs.
Signed-off-by: NPaul Gortmaker <paul.gortmaker@windriver.com>
Signed-off-by: NPaul E. McKenney <paulmck@linux.vnet.ibm.com>
Tested-by: NNicholas Piggin <npiggin@gmail.com>

Documentation/RCU/whatisRCU.txt

+What is RCU?  --  "Read, Copy, Update"
+
 Please note that the "What is RCU?" LWN series is an excellent place
 to start learning about RCU: