Documenting the MWI_HOST variable, whose default changed in v0.3.0

[MaximeWeyl]

I propose some more documentation on this environment variable, because :

• it was not documented at all
• its default changed from "" (all interfaces) to "127.0.0.1", which broke any connection from the outside (for instance, a container with an exposed 8888 port was rejecting connections).

Another solution would be to put the default back to 0.0.0.0. But having the default value being the more constrained/secured seems right to me.