syscall: clarify Pdeathsig documentation on Linux
This is a rather large footgun, so let's mention that it sends the signal on thread termination and not process termination in the documentation.
Updates #27505
Change-Id: I489cf7136e34a1a7896067ae24187b0d523d987e
GitHub-Last-Rev:
c8722b25d1fb8b0b3696257ec7e955eb421f15a6
GitHub-Pull-Request: golang/go#53365
Reviewed-on: https://go-review.googlesource.com/c/go/+/412114
Reviewed-by: Ian Lance Taylor <iant@google.com>
Auto-Submit: Ian Lance Taylor <iant@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Run-TryBot: Ian Lance Taylor <iant@google.com>
Reviewed-by: Cherry Mui <cherryyz@google.com>