Clarify usage of `<<` and `>>`

[tschmidtb51]

In my reading of vers, << and >> are not valid vers comparators. However, they are mentioned at the versioning scheme deb. Could you please clarify that?

[milux]

Second this. I think that the link provided is also not very helpful and may point to https://www.debian.org/doc/debian-policy/ch-controlfields.html#version instead.

The interesting question for me is: Do vers implementations need a from_native functionality for supported schemes that "translates" such ranges into vers-compatible ones? (I talk about the operators here, specifically.)