Update docs to specify that the termination signal is a SIGTERM

[copyconstruct]

Thee docs suggest that when the kill_timeout is specified, a task is sent a SIGINT. However, I'm seeing my task get a SIGTERM.

This comment by @dadgar also suggests that it's a SIGTERM that gets sent to a task.

[dadgar]

@copyconstruct Hey,

Sorry for any confusion! A SIGINT is sent first and if it does not exit, after the kill timeout the following is called: https://golang.org/pkg/os/#Process.Kill.

On posix systems that is defined to be a SIGKILL (https://golang.org/src/os/exec_posix.go)

[multani]

@copyconstruct Also see #2716 (in case you are using Docker)

[copyconstruct]

@multani Ah, yes, I figured that out myself. I run both Docker containers using the Docker driver and static binaries using the raw_exec driver, but the Dockerized app was only ever receiving a SIGTERM, when it was expecting a SIGINT to start the graceful shutdown. This wasn't a problem with my static binary. After adding the line STOPSIGNAL SIGINT to my Dockerfile, everything works as expected. But thanks for linking me to that PR.

@dadgar This was exactly the problem I was running into. It'd be great if the docs update was pushed out to the site. I spent a good few hours scratching my head as to why this wasn't working.

Thanks!

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