Try to fix incorrect documentation of nthreads (JuliaLang#53117)

Since JuliaLang#49094, the docstring of
`nthreads` has been incorrect. It currently states that

> The threads in default have id numbers `1:nthreads(:default)`.

whereas that is no longer true:
```julia
julia> filter(i -> Threads.threadpool(i) == :interactive, 1:Threads.maxthreadid())
3-element Vector{Int64}:
 1
 2
 3

julia> filter(i -> Threads.threadpool(i) == :default, 1:Threads.maxthreadid())
6-element Vector{Int64}:
 4
 5
 6
 7
 8
 9
```

(cherry picked from commit 95ae27f)

base/threadingconstructs.jl

@@ -44,8 +44,9 @@ maxthreadid() = Int(Core.Intrinsics.atomic_pointerref(cglobal(:jl_n_threads, Cin
 """
     Threads.nthreads(:default | :interactive) -> Int
 
-Get the current number of threads within the specified thread pool. The threads in default
-have id numbers `1:nthreads(:default)`.
+Get the current number of threads within the specified thread pool. The threads in `:interactive`
+have id numbers `1:nthreads(:interactive)`, and the threads in `:default` have id numbers in
+`nthreads(:interactive) .+ (1:nthreads(:default))`.
 
 See also `BLAS.get_num_threads` and `BLAS.set_num_threads` in the [`LinearAlgebra`](@ref
 man-linalg) standard library, and `nprocs()` in the [`Distributed`](@ref man-distributed)