[docs] clarify bundle feature version constraint

[gz]

Thanks for the library and maintaining it!

It might be good to clarify somewhere that the bundled feature works only when the exact version being bundled is selected with a = in front (at least that's what I needed to make it work):

during build:

POSTGRESQL_VERSION=15.13.0

and then

    let pg_settings = Settings {
        version: VersionReq::from_str(format!("={}", env!("POSTGRESQL_VERSION")).as_str())?,
        ..Default::default()
    };

[brianheineman]

@gz thanks for opening an issue. I created a PR to improve the documentation for this feature.