cpp: fix documentation of return value of GetNullaryOption() function

GetNullaryOption() is a function whose return type is bool, which can
only return two possible values, true or false, and thus can not return NULL.

--
Change-Id: If9fc622d7132b15268bdcc76aed8d04fe5b92bc7
Reviewed-on: https://bazel-review.googlesource.com/#/c/4140/2
MOS_MIGRATED_REVID=128448779

src/main/cpp/blaze_util.h

diff --git a/src/main/cpp/blaze_util.h b/src/main/cpp/blaze_util.h
index 4b477d3..68fd9bb 100644
--- a/src/main/cpp/blaze_util.h
+++ b/src/main/cpp/blaze_util.h
@@ -80,7 +80,7 @@
 
 // Returns true iff 'arg' equals 'key'.
 // Dies with a syntax error if arg starts with 'key='.
-// Returns NULL otherwise.
+// Returns false otherwise.
 bool GetNullaryOption(const char *arg, const char *key);
 
 // Enable messages mostly of interest to developers.