Add documentation of SubString

[bkamins]

SubString first appears in the section of regular expressions, but it is not mentioned earlier. I suggest to add documentation of SubString as it is returned by several functions from the standard library.

[KristofferC]

I was going to suggest adding a @ref to a SubString but it seems there is no corresponding docstring for SubString.

[nalimilan]

Thanks! I find it more natural to add this description after the presentation of getindex, at the end of "String Basics", don't you? The end of the section is already a good transition since it explains that chars and strings are very different. You could start saying that indexing creates a copy by default, but that there's a way to avoid it.

[bkamins]

@nalimilan moved, I agree that it is better placed here.

[nalimilan]

Hmm, one problem is that getindex isn't presented for strings. Maybe just say that indexing in the examples presented above make a copy, but that one can also use SubString. Also, it would make more sense IMHO to say this before the examples: else, it's not clear what a "view" is nor why one would want to create a view.

[bkamins]

This is what I also thought after committing - I hope the current wording is clear. Sorry for the confusion.

[nalimilan]

Thanks, looks good! If you want to add a docstring for SubString in another PR, you're welcome!

[bkamins]

I would add SubString documentation after #22511 is merged.

[rfourquet]

As String are (supposed to be) immutable, I was wondering whether it would make sense to remove the SubString type altogether, and handle copy vs substring at a lower level (in C?), invisible for the user?

[bkamins]

I am not an expert on C internals of Julia, but SubString can accept any AbstractString so I believe it could be difficult in general.