Document the AbstractString interface

[ararslan]

Currently in the Interfaces section of the manual, we lay out interfaces for iteration and AbstractArrays, but not AbstractStrings. Julia's String type is powerful and comprehensive enough that there's not really a reason to have a proliferation of AbstractString subtypes, but for those that do exist, it would be particularly helpful to have for porting code from 0.6, as the set of necessary functions has changed.

[quinnj]

Hear hear! I just ran into a case this morning w/ WeakRefStrings where we were getting a stackoverflow error because we forgot to implement thisind/nextind.

[samoconnor]

Related discussion: https://discourse.julialang.org/t/what-is-the-interface-of-abstractstring/8937

[samoconnor]

Also related: #26202

[adienes]

Any update here? I'm experimenting with a new formattable string type and I'd love it to be able to compose well with other strings, but it's not clear to me what that actually entails implementation-wise.

[tpapp]

@StefanKarpinski, as the original author for most of the string interface, would you please consider revisiting this if time permits? The source has a ## required string functions ## comment but it is not clear where that ends, and what is the minimum API one can get away with.