use of the `Base`-internal function `tuple_type_tail`

[Neven Sajko]

Base.tuple_type_tail is not part of Julia's public API:

Documenter.jl/src/cross_references.jl

Line 349 in 9c4fdb0

 getsig(λ::Union{Function, DataType}, typesig) = Base.tuple_type_tail(which(λ, typesig).sig)