gh-123905: Update TOML description to include version number (GH-123906)

Update TOML description to include version number

There is some movement, currently blocked, that would update the TOML spec to 1.1.0; this would include breaking changes to what characters are allowed. Thus, it is worthwhile for the library page to be clear which version is implemented here.

Co-authored-by: Paul Hoffman <phoffman@proper.com>