docs: standardize roxygen tag @return to @returns across R/ (#1363)

[wangzhengdna-lang]

Issue

Closes #1363

Description

Project convention (AGENTS.md, and the majority of the codebase) is the roxygen tag @returns, but 12 files still used @return (31 occurrences). This PR is a pure find-replace to @returns, plus one line wrap in R/l_pkcl01.R where the extra character pushed a doc line past the 100-character lint limit.

Files touched: R/exploration_plots.R, R/pk_dose_qc_plot.R, R/filter_breaks.R, R/PKNCA_extra_parameters.R, R/utils-slope_selector.R, R/PKNCA.R, R/flexible_violinboxplot.R, R/utils-plots.R, R/label_operators.R, R/export_cdisc.R, R/get_session_code.R, R/l_pkcl01.R.

No man/ changes: both tags generate identical \value{} sections, so the regenerated documentation is byte-identical.

Definition of Done

• No @return (without the s) remains in R/
• devtools::document() run to regenerate man/ (no output changes)
• lintr and tests pass

How to test

Documentation-only change; no behavior affected. Verified: zero @return (without s) left in R/, lintr::lint_package() 0 lints, full test suite 2005 PASS / 0 FAIL, spellcheck clean.

Contributor checklist

• Code passes lintr checks
• Code passes all unit tests
• New logic covered by unit tests
• New logic is documented
• App or package changes are reflected in NEWS
• Package version is incremented
• R script works with the new implementation (if applicable)
• Settings upload works with the new implementation (if applicable)
• If any .scss change was done, run data-raw/compile_css.R
• If a package dependency was added/changed, run data-raw/test_suggests_hidden.R

Notes to reviewer

Docs regenerated under the project RoxygenNote 7.3.3 standard produce no man/ diff, so the PR contains only the roxygen source edits.

[wangzhengdna-lang]

Follow-up commit 662d19f fixes the Man Pages / Roxygen CI failure.

Root cause: the line wrap I made in R/l_pkcl01.R (to stay within the 100-character lint limit) changed the generated \value{} text in man/l_pkcl01.Rd — any roxygen source change, even a pure re-wrap, alters the Rd output. The Roxygen check (7.3.3) regenerated man/ and found the file out of sync with the committed version.

Fix: regenerated man/l_pkcl01.Rd under the project's RoxygenNote 7.3.3 standard; the diff matches the CI expectation exactly. No other changes.