Standard library documentation is indeed clunky as it is auto-generated for the most part. This is something the community has been working on improving but it isn't a priority at this stage in part because the standard library undergoes breaking changes pretty frequently right now.

Optional and ErrorUnion are a tiny bit redundant in that one could represent the Optional as another value in an ErrorUnion, and that might even happen as an optimization step in the case of ?!/!? types at some point in the future, but they have very different handling in the language as they are used for very different things.

I personally like that Zig doesn't bother with "strings" at a language level at all and just considers everything as arrays of bytes. String handling is a complexity nightmare and I feel that Zig wisely chooses to be simple instead.