Expand description of ArchFamily, because it seems to be a common
source of confusion.  Also, update InFamily's description to reflect
current name.

Change-Id: I66b7999aef64ab8fee39aec0f752ae4f3a08d36d
Reviewed-on: https://go-review.googlesource.com/22102Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>