Instead of ambiguously referring to "the Client's CheckRedirect
function" in Head, describe the default behavior like for Get as users
aren't expected to change DefaultClient.CheckRedirect.

While here, use consistent punctuation for the Get and Head Client
method documentation.

Change-Id: I9e7046c73b0d0bc4de002234924d9e7c59aceb41
Reviewed-on: https://go-review.googlesource.com/11362Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>