Make the `Supported version syntax` section of `README.md` more precise?

[shindere]

As I understand it there are two points which would deserve clarification:

1. If a version number, say 5.2 is written without quotes, then it will be understood as a float, which the action's runtime will silently convert into a string, which would here be interpreted as 5, meaning that 5.3 would be a valid candidate. So one should write either 5.2.x (no quotes but an explicit x), or "5.2" since this time the quoted 5.2 will be interpreted as a string and understood as beeing equivalent to 5.2.x.
2. As stated in the documentation referred to by README.md:

Any of `X`, `x`, or `*` may be used to "stand in" for one of the numeric values in the `[major, minor, patch]` tuple.

In other words, in principle nothing guarantees that one will get the most recent possible version of the compiler. It seems this is the behaviour of the current implementation, though, but can one rely on it and, if so, could this be made explicit in the documentation?

All the credits for this issue should go to @MisterDA and @emillon who helped understanding the various aspects of this.

[smorimoto]

As implemented here: https://github.com/ocaml/setup-ocaml/blob/master/packages/setup-ocaml/src/version.ts

1. Get all the compilers from the packages/ocaml-base-compiler directory in the opam-repository.
2. Parse the compiler package name as semver to safely (but very optimistically!) create an intersection version string that conforms to semver and OCaml versioning.
3. If the received compiler version is semver-compatible, resolve the latest version from the user input version string and the compiler version list, based on semver.maxSatisfying. If not semver, optimistically just use it as the opam package.