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>

Documentation/dev-tools/sparse.rst

@@ -103,3 +103,9 @@ have already built it.
 
 The optional make variable CF can be used to pass arguments to sparse.  The
 build system passes -Wbitwise to sparse automatically.
+
+Checking RCU annotations
+~~~~~~~~~~~~~~~~~~~~~~~~
+
+RCU annotations are not checked by default.  To enable RCU annotation
+checks, include -DCONFIG_SPARSE_RCU_POINTER in your CF flags.