kbuild: clarify "make C=" build option

Clarify the use of "make C=" in the top-level Makefile, and fix a
typo in the Documentation file.
Signed-off-by: Sam Ravnborg <sam@ravnborg.org>

Documentation/sparse.txt

@@ -69,10 +69,10 @@ recompiled, or use "make C=2" to run sparse on the files whether they need to
 be recompiled or not.  The latter is a fast way to check the whole tree if you
 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.  To perform endianness
-checks, you may define __CHECK_ENDIAN__:
+The optional make variable CHECKFLAGS can be used to pass arguments to sparse.
+The build system passes -Wbitwise to sparse automatically.  To perform
+endianness checks, you may define __CHECK_ENDIAN__:
 
-        make C=2 CF="-D__CHECK_ENDIAN__"
+        make C=2 CHECKFLAGS="-D__CHECK_ENDIAN__"
 
 These checks are disabled by default as they generate a host of warnings.

Makefile

@@ -41,9 +41,15 @@ ifndef KBUILD_VERBOSE
   KBUILD_VERBOSE = 0
 endif
 
-# Call checker as part of compilation of C files
-# Use 'make C=1' to enable checking (sparse, by default)
-# Override with 'make C=1 CHECK=checker_executable CHECKFLAGS=....'
+# Call a source code checker (by default, "sparse") as part of the
+# C compilation.
+#
+# Use 'make C=1' to enable checking of only re-compiled files.
+# Use 'make C=2' to enable checking of *all* source files, regardless
+# of whether they are re-compiled or not.
+#
+# See the file "Documentation/sparse.txt" for more details, including
+# where to get the "sparse" utility.
 
 ifdef C
   ifeq ("$(origin C)", "command line")