write documentation for string.h functions.

src/string.h

@@ -2,7 +2,25 @@
 #include "atrb.h"
 #include "types.h"
 
-int   memcmp(const void *s1, const void *s2, usize n) NONNULL((1, 2)) PURE;
+/* Compares n bytes of s1 and s2.
+ * Returns =0 if both are equivalent,
+ * >0 if s1's value is greater, <0 if s2's value is greater.
+ * Returns 0 if n=0. */
+int memcmp(const void *s1, const void *s2, usize n) NONNULL((1, 2)) PURE;
+
+/* Sets n bytes of s to c.
+ * c shall be interpreted as an 8 bit unsigned integer.
+ * s is returned. */
 void *memset(void *s, int c, usize n) NONNULL((1));
+
+/* Copies n bytes from src to dst.
+ * src and dst may not alias, i.e. they mustn't overlap.
+ * If they do, use memmove instead.
+ * Returns dst. */
 void *memcpy(void *restrict dst, const void *restrict src, usize n) NONNULL((1, 2));
+
+/* Copies n bytes from src to dst.
+ * src and dst may alias one another, and will behave as if copied to an array.
+ * If src and dst can be guaranteed to not alias, memcpy is preferred.
+ * Returns dst. */
 void *memmove(void *dst, const void *src, usize n) NONNULL((1, 2));