Document `EvalAt`

[asinghvi17]

ModelingToolkit.jl/src/variables.jl

Lines 581 to 583 in 94b5775

	struct EvalAt <: Symbolics.Operator
	t::Union{Symbolic, Number}
	end

has no documentation, can we get some in? what does t mean here, for example - is that absolute or relative? is this autodiffable even if t is a symbolic?