doc/signals.rdoc: Clarify a bit where Signal.trap handlers are executed

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@67465 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

Author: eregon

doc/signals.rdoc

@@ -18,7 +18,7 @@ for data structures in YOUR code.  Ruby implements deferred signal
 handling by registering short C functions with only
 {async-signal-safe functions}[http://man7.org/linux/man-pages/man7/signal-safety.7.html] as
 signal handlers.  These short C functions only do enough tell the VM to
-run callbacks registered via Signal.trap later in the main VM loop.
+run callbacks registered via Signal.trap later in the main Ruby Thread.
 
 == Unsafe methods to call in Signal.trap blocks