doc: mention unit for process.hrtime.bigint()

The example states this already, but I would have expected this to be
part of the description, too.

PR-URL: #29482
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Michaël Zasso <targos@protonmail.com>
Reviewed-By: David Carlier <devnexen@gmail.com>
Reviewed-By: Rich Trott <rtrott@gmail.com>
Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>
Reviewed-By: Benjamin Gruenbaum <benjamingr@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>

Author: addaleax

doc/api/process.md

@@ -1339,7 +1339,7 @@ added: v10.7.0
 * Returns: {bigint}
 
 The `bigint` version of the [`process.hrtime()`][] method returning the
-current high-resolution real time in a `bigint`.
+current high-resolution real time in nanoseconds as a `bigint`.
 
 Unlike [`process.hrtime()`][], it does not support an additional `time`
 argument since the difference can just be computed directly