Add some comments to compiler.h

paddle/utils/Compiler.h

@@ -10,7 +10,10 @@ See the License for the specific language governing permissions and
 limitations under the License. */
 
 #pragma once
-
+/**
+ * This header defines some useful attribute by each compiler. It is the
+ * abstract layer of compilers.
+ */
 #ifdef __GNUC__
 #define GCC_VERSION \
   (__GNUC__ * 10000 + __GNUC_MINOR__ * 100 + __GNUC_PATCHLEVEL__)
@@ -18,6 +21,11 @@ limitations under the License. */
 #define GCC_VERSION
 #endif
 
+/**
+ * __must_check macro. It make the function's return value must be used,
+ * otherwise it will raise a compile warning. And also Paddle treat all compile
+ * warnings as errors.
+ */
 #if GCC_VERSION >= 30400
 #define __must_check __attribute__((warn_unused_result))
 #else