Add passing styles to single type return (#1274)

Resolves #1205  Suggestion 1, suggest making new elements for 2 and 3.

Signed-off-by: gregmarr <[email protected]>

docs/cpp2/functions.md

@@ -78,7 +78,7 @@ wrap_f: (
 
 A function can return either of the following. The default is `#!cpp -> void`.
 
-(1) **`#!cpp -> X`** to return a single unnamed value of type `X`, which can be  `#!cpp void` to signify the function has no return value. If `X` is not `#!cpp void`, the function body must have a `#!cpp return /*value*/;` statement that returns a value of type `X` on every path that exits the function.
+(1) **`#!cpp -> X`** to return a single unnamed value of type `X` using the same passing styles from the [parameters](#parameters) syntax, but where the only passing styles are `move` (the default) or `forward`.  The type can be  `#!cpp void` to signify the function has no return value. If `X` is not `#!cpp void`, the function body must have a `#!cpp return /*value*/;` statement that returns a value of type `X` on every path that exits the function.
 
 To deduce the return type, write `-> _`. A function whose body returns a single expression `expr` can deduce the return type, and omit writing the leading `-> _ = { return` and trailing `; }`.