clarify EMCC_LOCAL_PORTS

Author: kripken

tools/system_libs.py

@@ -794,11 +794,12 @@ class State:
     def retrieve():
       # if EMCC_LOCAL_PORTS is set, we use a local directory as our ports. This is useful
       # for testing. This env var should be in format
-      #     name=dir|subdir,name=dir|subdir
+      #     name=dir|tag,name=dir|tag
       # e.g.
       #     sdl2=/home/username/dev/ports/SDL2|SDL2-version_5
       # so you could run
       #     EMCC_LOCAL_PORTS="sdl2=/home/alon/Dev/ports/SDL2|SDL2-version_5" ./tests/runner.py browser.test_sdl2_mouse
+      # note that tag must be the tag in sdl.py, it is where we store to (not where we load from, we just load the local dir)
       local_ports = os.environ.get('EMCC_LOCAL_PORTS')
       if local_ports:
         local_ports = map(lambda pair: pair.split('='), local_ports.split(','))