doc: note that listening on SIGSEGV & co is unsafe

Note that trying to listen for some signals using `process.on()`
is unsafe in the `process` docs.

PR-URL: https://github.com/nodejs/node/pull/8410


Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>