[docs] lexical_analysis: Expand the text on ``_`` (GH-28903)

Also:
* Expand the discussion into its own entry. (Even before this,
  text on ``_`` was longet than the text on ``_*``.)

* Briefly note the other common convention for `_`: naming unused
  variables.

Co-authored-by: Brandt Bucher <brandtbucher@gmail.com>
Co-authored-by: Ken Jin <28750310+Fidget-Spinner@users.noreply.github.com>
Co-authored-by: Łukasz Langa <lukasz@langa.pl>