doc: describe unix signal handling better
doc: describe unix signal handling better

- SIGTERM seems OK to handle - I tested that on Linux, read POSIX, etc.
  I can't see why it should be different from SIGUSR1, for example,
  except that the default handler is different.
- Also raise(15) is caught by the handler for me, and I can't see
  why it wouldn't.

The main source of knowledge for this is POSIX.
http://pubs.opengroup.org/onlinepubs/9699919799/functions/V2_chap02.html#tag_15_04_03_03

PR-URL: https://github.com/libuv/libuv/pull/1987
Reviewed-By: Ben Noordhuis <59bd0a3ff43b32849b319e645d4798d8a5d1e889@bnoordhuis.nl>

unknownbrackets/maxcso,unknownbrackets/maxcso,unknownbrackets/maxcso,unknownbrackets/maxcso,unknownbrackets/maxcso,unknownbrackets/maxcso,unknownbrackets/maxcso