Docs: clarify selectInput() NULL handling with multiple = TRUE

[samuelbharti]

Fixes #3844

Summary

Extends the @note block in selectizeInput() and varSelectizeInput() (which @rdname into selectInput.Rd and varSelectInput.Rd, respectively) to call out a behavior that has surprised users: when multiple = TRUE, deleting all selected options yields an input value of NULL, and observeEvent() / eventReactive() ignore NULL by default — so the transition to "no selection" silently fails to trigger downstream reactives. The new note points users to ignoreNULL = FALSE as the knob to use when that behavior is undesirable.

This is a docs-only change. It follows the approach @gadenbuie outlined in #3844 and revives the work @BajczA475 started in the closed #3846, with corrected wording (ignoreNULL = FALSE, since the default is already TRUE).

No NEWS.md entry was added — recent precedent in this repo (e.g. f55c26a, 4361f2c, 5b6c187) is that pure roxygen clarifications don't get a NEWS bullet; NEWS is reserved for observable behavior changes. Happy to add one if preferred.

Verification

• devtools::document() regenerated man/selectInput.Rd and man/varSelectInput.Rd cleanly.
• Render ?selectInput and ?varSelectInput; the new paragraph appears under "Note" with working cross-references to observeEvent() and eventReactive() (the latter resolves via the alias in observeEvent.Rd).
• No code paths were touched, so no test changes were needed.

[samuelbharti]

Note for reviewers: the routine / routine check failed with an Invalid GitHub PAT format error inside devtools::document()'s reexport-tag-lookup step (gh::gh("GET /repos/.../tags") → validate_gh_pat()). This is unrelated to the docs change in this PR — all R-CMD-check jobs and website / pkgdown passed cleanly. PR #4386 from this same fork passed the same check yesterday, so this looks like a flaky/transient PAT-validation issue in fork-PR runs. A rerun of the failed job should clear it.

[BajczA475]

Rats, sorry, I forgot to finish up my work on this! Thanks to whomever finished it. Q1 26 was a hectic time for me :). Cheers to all of Shiny's maintainers!