SH-2738 Add comment documenting need to not make main thread wait on curl_multi_perform

indra/llmessage/llcurl.cpp

@@ -675,6 +675,10 @@ bool LLCurl::Multi::doPerform()
 				call_count++)
 		{
 			LLMutexLock lock(mMutexp) ;
+
+			//WARNING: curl_multi_perform will block for many hundreds of milliseconds
+			// NEVER call this from the main thread, and NEVER allow the main thread to 
+			// wait on a mutex held by this thread while curl_multi_perform is executing
 			CURLMcode code = curl_multi_perform(mCurlMultiHandle, &q);
 			if (CURLM_CALL_MULTI_PERFORM != code || q == 0)
 			{