docs: Show function signature of `BoundedLinear`

[korenmiklos]

This should be $(SIGNATURES)

SpectralKit.jl/src/transformations.jl

Line 19 in 881a6b8

$(TYPEDEF)

[tpapp]

Hmm, I am not quite sure about that, AFAIK $SIGNATURES only resolves for methods.

What is the actual issue this is supposed to fix?