[liba] Update the size_t definition

Change-Id: I88bf9b82956c471e5da53525e98af89bfaab5d48

liba/include/stddef.h

@@ -2,6 +2,11 @@
 #define LIBA_STDDEF_H
 
 #define NULL 0
+
+/* C99 says that size_t is an unsigned integer type of at least 16 bit.
+ * Moreover, size_t is the return type of the "sizeof" operator, which is
+ * defined by the compiler. That's why both GCC and clang define a __SIZE_TYPE__
+ * macro to let the libc header know about the type used by the compiler. */
 typedef __SIZE_TYPE__ size_t;
 
 #endif