update docs to address concerns

[Lokathor]

Updates the docs to help clarify the macro's operation.

• Closes Crate is not clear enough about cfg-if being limited to items #8
• Closes Expected fn not fn blah #10
• Closes else can be used without if #23

[alexcrichton]

Given a recent idea I think that should be tried out before documenting it only works with items, and I would prefer to not document quirks like "you can use else without anything else", I think it's fine to just leave that there as a nice nugget for anyone willing to figure it out.

[Lokathor]

Ah! The discussion was in a closed issue. Okay.