Document execve() limitation on Windows

Closes #1253

libc/proc/execve.c

@@ -57,6 +57,11 @@
  * compiled by MSVC or Cygwin is launched instead, then only the stdio
  * file descriptors can be passed along.
  *
+ * On Windows, the parent process must be a cosmo program. If you're
+ * calling execve() from a program that wasn't launched by cosmopolitan
+ * bash, or some similar program, then ask yourself if what you really
+ * want is to either (a) call fork() first, or (b) use posix_spawn().
+ *
  * On Windows, `argv` and `envp` can't contain binary strings. They need
  * to be valid UTF-8 in order to round-trip the WIN32 API, without being
  * corrupted.