ssh/terminal: use "reports whether" in IsTerminal doc

Go documentation style for boolean funcs is to say:

    // Foo reports whether ...
    func Foo() bool

(rather than "returns true if")

Change-Id: I6972d123ba99bbf3dbf95e876b45b2ecd98dd07c
Reviewed-on: https://go-review.googlesource.com/c/151257


Run-TryBot: Tobias Klauser <tobias.klauser@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>