doc: fs.readFile is async but not partitioned

This change was suggested during the discussion of #17054.

PR-URL: https://github.com/nodejs/node/pull/17154


Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Rich Trott <rtrott@gmail.com>
Reviewed-By: Gireesh Punathil <gpunathi@in.ibm.com>
Reviewed-By: Vse Mozhet Byt <vsemozhetbyt@gmail.com>