Go's http1 implementation originally had a mechanism to send HTTP
trailers based on pre-declaring the trailer keys whose values you'd
later let after the header was written.

http2 copied the same mechanism, but it was found to be unsufficient
for gRPC's wire protocol. A second trailer mechanism was added later
(but only to http2) for handlers that want to send a trailer without
knowing in advance they'd need to.

Copy the same mechanism back to http1 and document it.

Fixes #15754

Change-Id: I8c40d55e28b0e5b7087d3d1a904a392c56ee1f9b
Reviewed-on: https://go-review.googlesource.com/32479Reviewed-by: Ian Lance Taylor <iant@golang.org>