D 2.0 Reference: document cast(immutable) etc.

[darraghcoy]

Minor language documentation / reference issue.
I found these flavours of cast() expressions being parsed by DMD:

cast()
cast(const)
cast(immutable)
cast(invariant)
cast(shared)
cast(shared const)
cast(const shared)
cast(wild)
cast(shared wild)
cast(wild shared)

At the moment only cast(Type) is documented here, no mention is given to the
other forms of cast:
http://www.digitalmars.com/d/2.0/expression.html#CastExpression