subjectlaltname documentation improvement

[jonasbn]

Description

As mentioned in #92, the documentation could be improved in regards to the SANs returned by the subjectaltname method, this is a first shot at a clarification

Type of change

Please delete options that are not relevant.

• [X] This change requires a documentation update

Checklist

• [X] My code follows the style guidelines of this project
• [X] I have performed a self-review of my own code
• [X] I have made corresponding changes to the documentation
• [X] My changes generate no new warnings
• [X] New and existing unit tests pass locally with my changes

Test / Development Platform Information

• Operating system and version
• Crypt::OpenSSL::X509 version
• Perl version
• OpenSSL version

Please see the issue template for a more information on provided the requested information.

[jonasbn]

@timlegge would it be possible for you review this minor change?