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: https://github.com/nodejs/node/pull/28754


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