describe @exportS3Method for non-owned generics and delayed method registration

[tanho63]

Closes #1032 and closes #1082.

Cleans up the section about exporting S3 methods for non-owned generics to recommend using exportS3Method in all scenarios (hard and soft dependencies).

• For generics owned by hard dependencies (ie Depends and Imports), this previously described a workflow that required importing the generic into the NAMESPACE so that the method could be exported via @export - it seems like this can be skipped and we can use @exportS3Method directly in this scenario.

• For generics owned by soft dependencies (ie Suggests), this described a methodology around registering S3 methods onLoad(), and suggested an s3_register() function that needed to be included in the package body. This is no longer required as of R 3.6, which now interprets the S3method() directive in NAMESPACE to automatically register the method on dependency load. This means we can use @exportS3Method for this scenario as well, and remove the section on conditional/delayed load entirely.

I'm unsure if it would be worth mentioning how to handle things for pre R3.6 (and hide the deleted section in a collapsed details block?) - R 3.6 is quite old now.

[hadley]

Nice improvement! I've added a few comments/questions.

[hadley]

Thank you!