Add usage details for Base.eachsplit

[logankilpatrick]

I am reading through the docs for [Base.eachsplit](https://docs.julialang.org/en/v1/base/strings/#Base.eachsplit) and right now its use case is not clear to me. I see that the type it returns is different than split, but the current example does not make it clear why I would want to use it instead of just split:

julia> split(a, ".")
2-element Vector{SubString{String}}:
 "Ma"
 "rch"

julia> collect(eachsplit(a, "."))
2-element Vector{SubString}:
 "Ma"
 "rch"