Rename aws_security_token to aws_session_token

The AWS CLI actually accepts both, but for the shared credentials
file, we only accept aws_session_token as the standardized name
for the session token part of temporary credentials.

We should be only documenting the aws_session_token to encourage
usage of that value over aws_security_token.
Update docs with shared credentials changes

In addition, the section on nested attributes was removed.
We still support this type of value, but the switching
to signature version 4 for Amazon S3 is now handled by
the endpoints configuration so users do not manually need
to set this value.
Merge branch 'update-conf-docs' into develop

* update-conf-docs:
  Update docs with shared credentials changes
  Rename aws_security_token to aws_session_token