Docs: julia_cmd: Document use of julia_cmd()[1]

[fonsp]

Resolve #48393

I don't want to take up too much time for this issue, so feel free to close/edit without comment. I'm already happy to have found the solution ☺️

[giordano]

Looks good to me, but I'd like someone else to confirm it's ok to set this in stone.

[IanButterworth]

Seems a shame to do all the work to construct the args then throw them away.
What about add a julia_cmd(args=false)?

[IanButterworth]

Or just

"""
    Base.julia_exepath()

Returns the path to the julia executable of the running process.
"""
julia_exepath() = joinpath(Sys.BINDIR, julia_exename())

...
function julia_cmd(julia=julia_exepath(); cpu_target::Union{Nothing,String} = nothing)

Happy to PR it if people approve

[fonsp]

@IanButterworth I don't think this will be used in performance-critical code, right?

The advantage of documenting julia_cmd()[1] is that it also works on past Julia versions :)

[IanButterworth]

That could be in the compat note

"""
    Base.julia_exepath()

Returns the path to the julia executable of the running process.

!!! compat julia 1.10
    This was added in Julia 1.10. In prior versions use `julia_cmd()[1]`
    
"""
julia_exepath() = joinpath(Sys.BINDIR, julia_exename())

...
function julia_cmd(julia=julia_exepath(); cpu_target::Union{Nothing,String} = nothing)

[IanButterworth]

We can also just merge this as is and that can be a follow-on

[fonsp]

All sounds good to me! :)