Clarify clean_doc build documentation

[LeonidasZhak]

Fixes #187.

This clarifies the build(clean_doc = ) documentation for the currently implemented branches:

• FALSE leaves inst/doc unchanged
• NULL uses Config/build/clean-inst-doc when present
• if still NULL, interactive sessions ask before removing files, while non-interactive sessions clean them

Checks:

• Rscript -e 'tools::checkRd("man/build.Rd")'\n- git diff --check\n- R CMD build --no-build-vignettes .\n- R_LIBS_USER=/tmp/cli-r-lib _R_CHECK_FORCE_SUGGESTS_=false R CMD check --no-manual --ignore-vignettes --no-build-vignettes --no-tests pkgbuild_1.4.8.9000.tar.gz

[gaborcsardi]

@LeonidasZhak Is this a purely LLM-generated pull request? Please stop opening LLM-generated pull requests on the r-lib and tidyverse repositories for now.

[LeonidasZhak]

You are right. This was an LLM-assisted PR, and I should have been more careful before opening it. My intention was to explore whether LLM-assisted tools could help identify and fix small issues more efficiently, but I now understand that this created review burden and was not the right way to contribute here.
I’m closing this PR and will stop opening LLM-generated PRs on r-lib and tidyverse repositories for now. Sorry for adding noise to your review queue, and thank you for maintaining pkgbuild.