crypto/x509: fix comments on certDirectories

CL 205237 allowed SSL_CERT_DIR to be a colon delimited list of
directories. In the case that SSL_CERT_DIR is unset, the change
also made certDirectories to all be loaded rather than stopping
after successfully reading at least one file from a directory.
This update fixes code comments on the certDirectories package
level variables to reflect current behavior.

Fixes #48808

Change-Id: Id92f875545272fc6205d9955d03ea7bf844f15eb
Reviewed-on: https://go-review.googlesource.com/c/go/+/354140


Reviewed-by: Emmanuel Odeke <emmanuel@orijtech.com>
Reviewed-by: Katie Hockman <katie@golang.org>
Run-TryBot: Emmanuel Odeke <emmanuel@orijtech.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Trust: Katie Hockman <katie@golang.org>