crypto/x509: better documentation for ParsePKIXPublicKey

crypto/x509: better documentation for ParsePKIXPublicKey

The existing documentation for ParsePKIXPublicKey is difficult to understand
and the return type of the parsed public key are not mentioned explicitly.

Descriptions about types of public key supported, as well as an example on
how to use type assertions to determine return type of a parsed public key
has been added.

Fixes #14355

Change-Id: Ib9561efb34255292735742c0b3e835c4b97ac589
Reviewed-on: https://go-review.googlesource.com/19757
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>