Add findlast and findprev examples to strings.md manual

[royalbhati]

Added documentation for findlast() function

[fredrikekre]

Thanks for the PR and welcome as a contributor to Julia!
May I suggest that we combine your paragraph with the previous one? For example something like

You can search for the index of a particular character using the
[`findfirst`](@ref) and [`findlast`](@ref) functions:

```jldoctest
julia> findfirst(isequal('o'), "xylophone")
4

julia> findlast(isequal('o'), "xylophone")
7

julia> findfirst(isequal('z'), "xylophone")

```

and something similar can be done to also include findprev in the next paragraph if you feel like adding that. Thoughts?

[royalbhati]

Thanks for the PR and welcome as a contributor to Julia!
May I suggest that we combine your paragraph with the previous one? For example something like

You can search for the index of a particular character using the
[`findfirst`](@ref) and [`findlast`](@ref) functions:

```jldoctest
julia> findfirst(isequal('o'), "xylophone")
4

julia> findlast(isequal('o'), "xylophone")
7

julia> findfirst(isequal('z'), "xylophone")

and something similar can be done to also include `findprev` in the next paragraph if you feel like adding that. Thoughts?

Yes that sounds good

[nalimilan]

"a three"

Maybe just remove the mention of the number of arguments, that's clear just by looking at the examples.