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

docs/libcurl/libcurl.3

@@ -194,6 +194,9 @@ object as the program starts up and the destructor as it terminates.  As the
 author of this libcurl-using module, you can make the constructor call
 \fIcurl_global_init(3)\fP and the destructor call \fIcurl_global_cleanup(3)\fP
 and satisfy libcurl's requirements without your user having to think about it.
+(Caveat: If you are initializing libcurl from a Windows DLL you should not
+initialize it from DllMain or a static initializer because Windows holds the
+loader lock during that time and it could cause a deadlock.)
 
 \fIcurl_global_init(3)\fP has an argument that tells what particular parts of
 the global constant environment to set up.  In order to successfully use any