doc: make minor improvements to fs.realpath() docs

Make canonical vs. unique text more precise and eliminate quotation
marks. Remove repeated text from fs.realpathSync(), linking to
fs.realpath() instead.

PR-URL: https://github.com/nodejs/node/pull/20953


Reviewed-By: Ruben Bridgewater <ruben@bridgewater.de>
Reviewed-By: Vse Mozhet Byt <vsemozhetbyt@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>