brute force documentation update

docs/emcc.txt

@@ -466,6 +466,12 @@ Options that are modified or new in *emcc* are listed below:
    By default this will also clear any download ports since the ports
    directory is usually within the cache directory.
 
+"--use-port=<port>"
+   [link] Use the specified port. If you need to use more than one
+   port you can use this argument multiple times. For example: "--use-
+   port=sdl2 --use-port=bzip2". To get the list of available ports,
+   use "--show-ports".
+
 "--clear-ports"
    [general] Manually clears the local copies of ports from the
    Emscripten Ports repos (sdl2, etc.). This also clears the cache, to