manual: point to the destructuring interface functions (`rest`, `split_rest`) from the Interfaces page

[nsajko]

The doc strings are already in the Manual, but the Interfaces page does not mention them:

• https://docs.julialang.org/en/v1/manual/interfaces/

• https://docs.julialang.org/en/v1/base/collections/#Base.rest

• https://docs.julialang.org/en/v1/base/collections/#Base.split_rest

They should be added to the Interfaces page, as user-defined collection types might wish to add methods for performance.

Side note: there's another function that part of the destructuring interface, although it is not public: indexed_iterate. Perhaps it, too, should be public?

Example of the lowering of destructuring:

julia> function f(x)
           (a, b, c...) = x  # lowers using `rest`
           (m, n..., p, q) = c  # lowers using `split_rest`
           n
       end
f (generic function with 1 method)

julia> code_lowered(f)
1-element Vector{Core.CodeInfo}:
 CodeInfo(
1 ─ %1  =   dynamic Base.indexed_iterate(x, 1)
│         a = Core.getfield(%1, 1)
│         @_4 = Core.getfield(%1, 2)
│   %4  = @_4
│   %5  =   dynamic Base.indexed_iterate(x, 2, %4)
│         b = Core.getfield(%5, 1)
│         @_4 = Core.getfield(%5, 2)
│   %8  = @_4
│         c = Base.rest(x, %8)
│   %10 = c
│   %11 =   dynamic Base.indexed_iterate(%10, 1)
│         m = Core.getfield(%11, 1)
│         @_3 = Core.getfield(%11, 2)
│   %14 = c
│   %15 = @_3
│   %16 =   dynamic Base.split_rest(%14, 2, %15)
│         n = Core.getfield(%16, 1)
│   %18 =   builtin Core.getfield(%16, 2)
│   %19 =   dynamic Base.indexed_iterate(%18, 1)
│         p = Core.getfield(%19, 1)
│         @_3 = Core.getfield(%19, 2)
│   %22 = @_3
│   %23 =   dynamic Base.indexed_iterate(%18, 2, %22)
│         q = Core.getfield(%23, 1)
│   %25 = n
└──       return %25
)