curl_global_init.3: Add Windows-specific info for init via DLL

- Add to both curl_global_init.3 and libcurl.3 the caveat for Windows
that initializing libcurl via a DLL's DllMain or static initializer
could cause a deadlock.

Bug: https://github.com/bagder/curl/issues/586


Reported-by:  <marc-groundctl@users.noreply.github.com>