Document link/compile/both options

Generally, every time I'm looking up one of the required Emscripten options for either thing I'm trying to do, in either part of the documentation or wiki, I have generally *no idea* whether it's required at **compile step**, at the **link step**, or **both**.

Right now, specifically, I'm looking at `-s SIDE_MODULE=1` and `-s MAIN_MODULE=1`, but this is basically the case for every single Emscripten option. It's virtually **never specified** at which step it's required, whereas ideally it would be **always specified**.

So the user ends up needing to fiddle with build scripts, do a full rebuild a bunch of time in order to determine that. If I want to correctly specify my build scripts, I basically am required to do **three full rebuilds** to determine at which step it was actually needed (and even then I still might not be entirely sure). This is really not ideal.

This is not the first time I'm deeply annoyed by this issue, so I thought I'd at least open an issue.