cleanup std.signals documentation

[dlangBugzillaToGithub]

greeenify reported this on 2016-12-22T16:56:17Z

Transfered from https://issues.dlang.org/show_bug.cgi?id=17011

Description

There's no need to explain the history in such a depth for a usual reader, better describe it's features and what makes it unique/better to other implementations (or shortcomings), e.g.:

https://code.dlang.org/packages/phobosx "Replacement for std.signals, with more features and less bugs"

https://code.dlang.org/packages/observe


Also adding a couple of good examples wouldn't hurt.