Since #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)

…52855)

There is no `C` defined in these methods, so this branch used to error.

(cherry picked from commit c5d7b87)

(cherry picked from commit a0d55cd)

Since the diagonal elements of a `UnitUpperTriangular` are given by
`onelement`, these should be unchanged under `transpose/adjoint`, and we
don't need to access these elements in the parent array when performing
in-place operations.

Fixes
```julia
julia> using LinearAlgebra

julia> M = Matrix{BigFloat}(undef, 2, 2);

julia> M[1,2] = 3;

julia> U = UnitUpperTriangular(M)
2×2 UnitUpperTriangular{BigFloat, Matrix{BigFloat}}:
 1.0  3.0
  ⋅   1.0

julia> transpose!(U)
ERROR: UndefRefError: access to undefined reference
Stacktrace:
 [1] getindex
   @ ./essentials.jl:882 [inlined]
 [2] getindex
   @ ./array.jl:915 [inlined]
 [3] copytri!
   @ ~/packages/julias/julia-latest/share/julia/stdlib/v1.11/LinearAlgebra/src/matmul.jl:414 [inlined]
 [4] transpose!(A::UnitUpperTriangular{BigFloat, Matrix{BigFloat}})
   @ LinearAlgebra ~/packages/julias/julia-latest/share/julia/stdlib/v1.11/LinearAlgebra/src/triangular.jl:470
 [5] top-level scope
   @ REPL[5]:1
```
After this PR:
```julia
julia> transpose!(U)
2×2 UnitLowerTriangular{BigFloat, Matrix{BigFloat}}:
 1.0   ⋅
 3.0  1.0
```

(cherry picked from commit cc74d24)