Improve documentation to explain where callback-functions run
Imported from http://bugzilla.roxen.com/bugzilla/show_bug.cgi?id=6932
Reported by Pontus Rodling frigolit@roxen.com
Signal callback defined in Process.create_process will run in any non-busy thread, which can cause some funny behavior when using for example mutexes. The same goes for calls to destroy()-functions triggered by gc. The documentation needs information about in what context these functions are called.