Improve sparse documentation

Add documentation of -DCONFIG_SPARSE_RCU_POINTER.
I started to add documentation of -D__CHECK_ENDIAN__ as well, but
discovered I'm too late; that's now enabled by default.

Signed-off-by: Matthew Wilcox <mawilcox@microsoft.com>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>