doc: expand documentation for process.exit()

The fact that process.exit() interrupts pending async operations
such as non-blocking i/o is becoming a bit more pronounced with
the recent libuv update. This commit expands the documentation
for `process.exit()` to explain clearly how it affects async
operations.

PR-URL: https://github.com/nodejs/node/pull/6410


Reviewed-By: Сковорода Никита Андреевич <chalkerx@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Robert Lindstaedt <robert.lindstaedt@gmail.com>