doc: revise deepEqual() legacy assertion mode text

* Remove "potentially". It's arguably misplaced and certainly
  unnecessary.
* Use italics for words-as-words usage of _Deep equality_ rather than
  using quotation marks.

PR-URL: https://github.com/nodejs/node/pull/31704


Reviewed-By: Ruben Bridgewater <ruben@bridgewater.de>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>