doc: slightly improve description of "key wrapping" in security document
doc: slightly improve description of "key wrapping" in security document

Fix #537

Change-Id: I01abe06f5d97a7a4f638a75739c980f67b51b8cc
Reviewed-on: https://upspin-review.googlesource.com/16380
Reviewed-by: Rob Pike <4dc7c9ec434ed06502767136789763ec11d2c4b7@golang.org>

upspin/upspin,upspin/upspin