Define nextind(x, i::Integer) or document existing methods

[jariji]

nextind has a bunch of methods, only the AbstractString one is documented. I think a generic nextind(x, i::Integer) would be good. If that's not accepted, at least the existing methods should be documented.

julia> methods(nextind)
# 13 methods for generic function "nextind" from Base:
  [1] nextind(source::JuliaSyntax.SourceFile, i::Integer)
     @ JuliaSyntax ~/.julia/packages/JuliaSyntax/eqCSU/src/source_files.jl:127
  [2] nextind(source::Base.JuliaSyntax.SourceFile, i::Integer)
     @ Base.JuliaSyntax /cache/build/builder-amdci4-6/julialang/julia-release-1-dot-10/base/JuliaSyntax/src/source_files.jl:127
  [3] nextind(s::String, i::Int64)
     @ strings/string.jl:171
  [4] nextind(s::SubString{String}, i::Int64)
     @ strings/substring.jl:104
  [5] nextind(s::AbstractString, i::Int64, n::Int64)
     @ strings/basic.jl:558
  [6] nextind(s::AbstractString, i::Integer, n::Integer)
     @ strings/basic.jl:554
  [7] nextind(s::AbstractString, i::Int64)
     @ strings/basic.jl:556
  [8] nextind(s::AbstractString, i::Integer)
     @ strings/basic.jl:555
  [9] nextind(::OrderedCollections.OrderedSet, i::Int64)
     @ OrderedCollections ~/.julia/packages/OrderedCollections/9C4Uz/src/ordered_set.jl:105
 [10] nextind(a::AbstractArray{<:Any, N}, i::CartesianIndex{N}) where N
     @ Base.IteratorsMD multidimensional.jl:149
 [11] nextind(::AbstractArray, i::Integer)
     @ abstractarray.jl:213
 [12] nextind(t::Tuple, i::Integer)
     @ tuple.jl:78
 [13] nextind(t::NamedTuple, i::Integer)
     @ namedtuple.jl:181

help?> nextind
search: nextind IndexCartesian MissingException current_exceptions InterruptException InvalidStateException

  nextind(str::AbstractString, i::Integer, n::Integer=1) -> Int


    •  Case n == 1
       If i is in bounds in s return the index of the start of the character whose encoding starts after index
       i. In other words, if i is the start of a character, return the start of the next character; if i is
       not the start of a character, move forward until the start of a character and return that index. If i
       is equal to 0 return 1. If i is in bounds but greater or equal to lastindex(str) return
       ncodeunits(str)+1. Otherwise throw BoundsError.

    •  Case n > 1
       Behaves like applying n times nextind for n==1. The only difference is that if n is so large that
       applying nextind would reach ncodeunits(str)+1 then each remaining iteration increases the returned
       value by 1. This means that in this case nextind can return a value greater than ncodeunits(str)+1.

    •  Case n == 0
       Return i only if i is a valid index in s or is equal to 0. Otherwise StringIndexError or BoundsError is
       thrown.

[nsajko]

1. On master, there's generic documentation due to Document the generic functions nextind() and prevind() #52658: https://docs.julialang.org/en/v1.12-dev/base/arrays/#Base.nextind
2. The mentioned PR left the Markdown broken, thus nextind, prevind doc strings: correct the Markdown #54876