FAQ: expand the thread-safe section

... with a mention of *NOSIGNAL, based on talk in bug #1386

docs/FAQ

@@ -1099,6 +1099,12 @@ FAQ
   your system has such.  Note that you must never share the same handle in
   multiple threads.
 
+  libcurl's implementation of timeouts might use signals (depending on what it
+  was built to use for name resolving), and signal handling is generally not
+  thread-safe.  Multi-threaded Applicationss that call libcurl from different
+  threads (on different handles) might want to use CURLOPT_NOSIGNAL, e.g.:
+
+    curl_easy_setopt(handle, CURLOPT_NOSIGNAL, true);
 
   If you use a OpenSSL-powered libcurl in a multi-threaded environment, you
   need to provide one or two locking functions: