Make minor improvements

libc/str/toupper.c

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