Tweak the definition of Subject in Glossary

http://openam.forgerock.org/openam-documentation/openam-doc-source/doc/admin-guide/index/openam-glossary.html

"Subject - Entity that can request access to a resource"

We spent some time pondering this whilst answering a question for a new starter. Perhaps it might be more useful to indicate that a Subject gains a Principal as a result of requesting access to a resource, and that a Subject can have multiple Principals.