Add note about schema url and schema version

[jack-berg]

It wasn't obvious to me that schema_url requires that the last part of the URL path is the schema version number.

From the spec:

The last part of the URL path is the version number of the schema.

This PR extends the comment for each schema_url to make this crucial piece of information clear. Note there are still other relevant bits of information about schema_url that readers need to follow the link to https://opentelemetry.io/docs/specs/otel/schemas/#schema-url in order to understand. E.g. that the version follows the format MAJOR.MINOR.PATCH and more.