doc: add information for heap snapshot flag

It's nice to have usage examples, especially since the flag requires the
`SIG` version of the signal name, unlike `kill`.

PR-URL: #28754
Reviewed-By: Rich Trott <rtrott@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>

Author: tstirrat15

doc/api/cli.md

@@ -223,6 +223,19 @@ reference. Code may break under this flag.
 added: v12.0.0
 -->
 
+Enables a signal handler that causes the Node.js process to write a heap dump
+when the specified signal is received.
+
+```console
+$ node --heapsnapshot-signal=SIGUSR2 index.js &
+$ ps aux
+USER       PID %CPU %MEM    VSZ   RSS TTY      STAT START   TIME COMMAND
+node         1  5.5  6.1 787252 247004 ?       Ssl  16:43   0:02 node --heapsnapshot-signal=SIGUSR2 index.js
+$ kill -USR2 1
+$ ls
+Heap.20190718.133405.15554.0.001.heapsnapshot
+```
+
 ### `--heap-prof`
 <!-- YAML
 added: v12.4.0