Add some details for non visible scrollspy elements (#36625)

* docs: add some details for non visible scrollspy elements

* Apply suggestions from code review

Co-authored-by: Julien Déramond <julien.deramond@orange.com>

* fix selectors on snippet

* Fix paragraph by a native speaker

Co-authored-by: Mark Otto <markd.otto@gmail.com>

* Update scrollspy.md

* Move from warning to own section

* Update scrollspy.md

Co-authored-by: Julien Déramond <julien.deramond@orange.com>
Co-authored-by: Julien Déramond <juderamond@gmail.com>
Co-authored-by: Mark Otto <markd.otto@gmail.com>
Co-authored-by: Mark Otto <markdotto@gmail.com>