doc: mention unit for event loop delay measurements

PR-URL: https://github.com/nodejs/node/pull/28629
Reviewed-By: Ben Coe <bencoe@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Gireesh Punathil <gpunathi@in.ibm.com>
Reviewed-By: Ruben Bridgewater <ruben@bridgewater.de>

doc/api/perf_hooks.md

@@ -419,7 +419,7 @@ added: v11.10.0
 * Returns: {Histogram}
 
 Creates a `Histogram` object that samples and reports the event loop delay
-over time.
+over time. The delays will be reported in nanoseconds.
 
 Using a timer to detect approximate event loop delay works because the
 execution of timers is tied specifically to the lifecycle of the libuv