Make minor improvements

libc/str/tolower.c

@@ -19,6 +19,9 @@
 ╚─────────────────────────────────────────────────────────────────────────────*/
 #include "libc/str/str.h"
 
+/**
+ * Converts character to lower case.
+ */
 int tolower(int c) {
   return 'A' <= c && c <= 'Z' ? c + ('a' - 'A') : c;
 }