Docstring of |>: add parentheses around -> func

[tfiers]

This clarifies the current behaviour where the subsequent pipes get parsed as part of the anonymous function's body if there are no parentheses.

The current line

[1:5;] |> x->x.^2 |> sum |> inv

is parsed as

[1:5;] |> x -> (x.^2 |> sum |> inv)

but it feels like the author meant (and readers would expect)

[1:5;] |> (x->x.^2) |> sum |> inv

This PR makes this explicit.

See also #38761 and #43661.