Docs: Missing documentation that `['*', 'x', 'X']` can all be wildcards

By accident, I discovered that `VersionReq` can parse all 3 of `['*', 'x', 'X']` characters as a wildcard, while it is not documented either here nor in [cargo rust docs](https://doc.rust-lang.org/cargo/reference/specifying-dependencies.html#version-requirement-syntax). Are there any difference between how they are used?

https://github.com/dtolnay/semver/blob/4ea60ae121bf0c8c5ad7e7aa5a9f663f305b7400/src/parse.rs#L191-L201

Start with the wildcard parsing logic in src/parse.rs around the linked lines, then compare it with Cargo's version requirement syntax documentation. Determine whether *, x, and X have any different behavior and document the result in the project's relevant documentation; done means the supported wildcard forms and their semantics are clear.

Written by the indexing model from the issue text.