HasEltype and EltypeUnknown should be public from Base?

[nsajko]

julia> Base.ispublic(Base, :HasEltype)
false

julia> Base.ispublic(Base, :EltypeUnknown)
false

julia> versioninfo()
Julia Version 1.12.0-DEV.1757
Commit 06f988b7415 (2024-12-14 21:20 UTC)
Build Info:
  Official https://julialang.org release
Platform Info:
  OS: Linux (x86_64-linux-gnu)
  CPU: 8 × AMD Ryzen 3 5300U with Radeon Graphics
  WORD_SIZE: 64
  LLVM: libLLVM-18.1.7 (ORCJIT, znver2)
Threads: 9 default, 0 interactive, 8 GC (on 8 virtual cores)

These are widely used and a part of the iteration interface. That said, they don't have doc strings, and although they're mentioned in the Manual, it was never completely clear to me what each of them means. As far as I gather, the only significance is that the choice affects the behavior of collect, and perhaps some similar functions in the ecosystem: EltypeUnknown indicates that eltype should not be relied on for collect.

[jishnub]

Irrespective of whether these are made public, we may certainly add docstrings.