Documentation for prevind/nextind on tuples and arrays is missing

[Seelengrab]

Currently only prevind(::AbstractString, i::Integer, n::Integer=1) -> Int is documented, but quite a few more methods exist:

julia> methods(prevind)                                                                                             
# 8 methods for generic function "prevind":                                                                         
[1] prevind(t::Tuple, i::Integer) in Base at tuple.jl:74                                                            
[2] prevind(::AbstractArray, i::Integer) in Base at abstractarray.jl:183                                            
[3] prevind(t::NamedTuple, i::Integer) in Base at namedtuple.jl:135                                                 
[4] prevind(s::AbstractString, i::Int64) in Base at strings/basic.jl:494                                            
[5] prevind(s::AbstractString, i::Int64, n::Int64) in Base at strings/basic.jl:496                                  
[6] prevind(s::AbstractString, i::Integer) in Base at strings/basic.jl:493                                          
[7] prevind(s::AbstractString, i::Integer, n::Integer) in Base at strings/basic.jl:492                              
[8] prevind(a::AbstractArray{<:Any, N}, i::CartesianIndex{N}) where N in Base.IteratorsMD at multidimensional.jl:158

These are generic enough to be documented as well, no? It's the most generic way to the get previous index of any abstract vector (though the default implementation assumes Integers are used for indexing).

The same applies to nextind - both were added for NamedTuples in #37932

[aplavin]

Also, nextind(A, i::Integer, n::Integer=1) methods would be nice for all supported types. Currently, 3-arg only works for strings, while vectors and tuples only have 2-arg (without n)`.