Auto-generate some documentation

libc/str/isalnum.c

@@ -19,6 +19,9 @@
 ╚─────────────────────────────────────────────────────────────────────────────*/
 #include "libc/str/str.h"
 
+/**
+ * Returns nonzero if c is lower, alpha, or digit.
+ */
 int isalnum(int c) {
   return ('0' <= c && c <= '9') || ('A' <= c && c <= 'Z') ||
          ('a' <= c && c <= 'z');