runtime: document signals caught and handled by the runtime

[nightlyone]

Various signals (e.g. SIGQUIT) are handled by the runtime and lead to special default actions in all Go programs.

It would be great to document these somewhere (e.g. os/signal) so people can find these magic signals and their expected/current behavior somewhere.

[mlafeldt]

Today I tried to remember the signal which causes a Go program to dump all goroutines. Even through googling it was hard to figure out that SIGQUIT is the right answer. So 👍 for adding this information to a well-known location.

[ianlancetaylor]

Good idea but I don't think os/signal is the right place for this. Setting the package to runtime for now.

[rsc]

FWIW the argument for putting these docs in os/signal (perhaps with a forward reference from package runtime) is that os/signal's funcs change the behavior. It's easier to describe both the default and modified behaviors in one place.

[rsc]

Dup of #9896.