doc: fs.readFile is async but not partitioned

This change was suggested during the discussion of #17054.

PR-URL: #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>

doc/api/fs.md

@@ -1897,6 +1897,11 @@ Any specified file descriptor has to support reading.
 *Note*: If a file descriptor is specified as the `path`, it will not be closed
 automatically.
 
+*Note*: `fs.readFile()` reads the entire file in a single threadpool request.
+To minimize threadpool task length variation, prefer the partitioned APIs
+`fs.read()` and `fs.createReadStream()` when reading files as part of
+fulfilling a client request.
+
 ## fs.readFileSync(path[, options])
 <!-- YAML
 added: v0.1.8