Clarify labelled vectors in read docs

[LeonidasZhak]

Fixes #741.

This updates the read_*() documentation to make the imported labelled-vector workflow more explicit for users coming from Stata, SPSS, or SAS:

• notes that value labels are preserved as labelled() / labelled_spss() intermediate representations, not regular R factors
• points users to as_factor() for labelled categorical variables
• points users to zap_labels() when plain R vectors are needed for analysis
• updates the generated Rd files and NEWS entry alongside the roxygen source

I kept this as a documentation-only change and did not add a runtime message, following the maintainer preference in #741.

Checks run locally:

• tools::checkRd() for man/read_dta.Rd, man/read_spss.Rd, and man/read_sas.Rd
• git diff --check
• R CMD build --no-build-vignettes .
• _R_CHECK_FORCE_SUGGESTS_=false R CMD check --no-manual --ignore-vignettes --no-build-vignettes --no-tests haven_2.5.5.9000.tar.gz

The local machine does not currently have suggested packages testthat and covr, so I could not run the full test suite locally. The no-tests check installs the package, checks Rd/examples, and completes with one existing NOTE about setNames that is unrelated to this documentation change.