Auto-generate some documentation

libc/str/isalpha.c

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