Clarify "type" in Resource semantic conventions

[lmolkova]

Resources have a "type" property (manually written, not part of yaml). E.g.

https://github.com/open-telemetry/semantic-conventions/blob/4728f6374361ad82949eecdfccb67759f9f2535c/docs/resource/README.md?plain=1#L74-L78

It's not clear what it means - so we should either document it or remove it.

Originally posted by @lmolkova in https://github.com/open-telemetry/semantic-conventions/pull/919#discussion_r1562678752