fix(r-cran-extrachecks): exempt @export-only S3 methods from docs checks

[gadenbuie]

Summary

Clarifies the r-cran-extrachecks skill's documentation rules so it stops flagging exported S3 methods that are documented only with #' @export as missing @return/@examples (reported in #63).

Roxygen2 only generates an .Rd file when a block contains documentation content. A block with only #' @export adds the function to the NAMESPACE without producing an .Rd, so CRAN's documentation checks have nothing to inspect. R CMD check's undocumented-objects check also excludes registered S3 methods, so the conventional print.my_class() pattern passes as-is.

What changed

• Return value documentation: added an "applies only to functions with an .Rd file" clarification with the print.my_class() example, plus guardrails:
  • The exemption holds only while the block contains nothing but @export/@exportS3Method — adding any documentation content (title, @param, @rdname, @describeIn) makes roxygen2 generate an .Rd and all rules apply again
  • Newly exported S3 generics are ordinary documented exported functions and still need @return/@examples
  • Methods sharing a page via @rdname/@describeIn follow the generic's rules — required tags once per .Rd
• Examples section: matching bullet pointing to the exemption
• Workflow step 5: notes the exemption during the initial documentation pass
• Checklist: reworded two items to key off .Rd files; added items for the S3-method exemption and S3-generics-with-full-docs checks

Skill remains within token limits (475 lines, ~3.9k tokens).

Fixes #63