crypto/x509: expand package docs and clarify package target

crypto/x509: expand package docs and clarify package target

Fixes #26624

Change-Id: Ifab3fc2209d71b9a7de383eaa5786b7446de25fa
Reviewed-on: https://go-review.googlesource.com/c/go/+/266541
Reviewed-by: Roland Shoemaker <roland@golang.org>
Run-TryBot: Roland Shoemaker <roland@golang.org>
Reviewed-by: Jonathan Amsterdam <jba@google.com>
Reviewed-by: Filippo Valsorda <filippo@golang.org>
Auto-Submit: Roland Shoemaker <roland@golang.org>
TryBot-Result: Gopher Robot <gobot@golang.org>