docs: advertise composition of polynomials

Searching the docs for "composition" or "substitution" doesn't turn up anything. The same code path designed for evaluating a polynomial seems to work fine for composing two polynomials together, though:
```julia-repl
julia> using Polynomials

julia> p = Polynomial([0, 1])
Polynomial(x)

julia> q = Polynomial(Float64[0, 0.5])
Polynomial(0.5*x)

julia> r = Polynomial(Float64[0, 2])
Polynomial(2.0*x)

julia> q(r) == r(q) == p == p(p) == p(p(p)) == p(p(p(p)))
```