Fix misleading comment on ELF.map (#1012)
Fix misleading comment on ELF.map (#1012)

Motivation:

We left a doc comment on EventLoopFuture.map that caused confusion by
misleadingly claiming that it could handle throwing functions. The
operation that handles throwing functions is flatMapThrowing.

Modifications:

- Removed the misleading comment and pointed users to flatMapThrowing
    instead.

Result:

Less user confusion.
apple/swift-nio,apple/swift-nio,apple/swift-nio,apple/swift-nio,apple/swift-nio