Document how the agent gracefully restarts about docs HOT 7 CLOSED

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Relevant to buildkite/agent#476

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Also some description of what signals we handle and some nice descriptions like here: https://www.quora.com/What-is-the-difference-between-the-SIGINT-and-SIGTERM-signals-in-Linux-What%E2%80%99s-the-difference-between-the-SIGKILL-and-SIGSTOP-signals

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This is the canonical documentation presently: https://github.com/buildkite/agent/blob/ce7280de5d8fc20850f899bd4a1b23e6ced995f2/agent/agent_pool.go#L81

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Also, I just really like leaving comments on this issue.

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@lox we have a small section on Signal Handling in the overview document of the Agent.

This section might be better with a little more context and in a different place - what do you think about an additional mention and a link to signal handling in the config doc and another in the buildkite-agent start doc? Otherwise I could just add a little more info to the current Signal Handling section so that the probable search terms like 'new config' or 'shutdown agents' will pop up in google with this section.

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👋🏻 @lox! Given that the 'stop agent' button is (somewhat recently) graceful and has the option for force stopping (https://github.com/buildkite/buildkite/pull/3064), do we need more docs additions about stopping agents? Possibly just a mention in the signal handling section that they can be killed from the Agent UI?

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