Briefly document what the importfoo functions do.

Get rid of importsym's unused result parameter.

Get rid of the redundant calls to importsym(s, OTYPE)
after we've already called pkgtype(s).

Passes toolstash -cmp.

Change-Id: I4c057358144044f5356e4dec68907ec85f1fe806
Reviewed-on: https://go-review.googlesource.com/21498
Run-TryBot: Matthew Dempsky <mdempsky@google.com>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Reviewed-by: Robert Griesemer <gri@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>