Bug 21362 – Unclear specification of `in` parameter storage class

At https://dlang.org/spec/function.html#param-storage it is unclear what the meaning of `in` is, because of the last sentence "This requires the -preview=in switch, available in v2.094.0 or higher." What does "This" refer to? Please reformulate, for example by: "The parameter is an input to the function. Input parameters behaves as if they have the const scope storage classes.... When `-preview=in` switch is passed to compiler, the semantics become: The parameter is an input to the function. Input parameters behaves as if they have the const scope storage classes.... "

Fixed in https://github.com/dlang/dlang.org/pull/2937 by making it the start of the section (and its own section).