Document options and defines related to .NET DLL support

I always forget about how/where haxe gets .NET dlls, so it would be nice to document that eventually, so we don't have to look into haxe sources to find out.

Here's a list of related options:
- -net-lib (and `@std` postfix)
- -net-std
- -D net-target
- -D net-ver
- NET_\* automatic defines

Hope I haven't forget anything.
@waneck How stable is this? Can we document it or it's subject to change?