dbt docs: clarify tenant part of student surrogate keys

[rlittle08]

hotfix/k_student_dbt_docs

PR Merge Priority:

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Changes to existing files:

• Update dim_student.yaml to be more precise on surrogate key defs for commonly referenced k_student and k_student_xyear.

edu_wh PR Review Checklist:

Make sure the following have been completed before approving this PR:

• Description of changes has been added to Unreleased section of CHANGELOG.md. Add under ## New Features for features, etc.
• If a new configuration xwalk was added:
  • The code is written such that the xwalk is optional (preferred), and this behavior was tested, OR
  • The code is written such that the xwalk is required, and the required xwalk is added to edu_project_template, and this PR is flagged as breaking change (not for patch release)
  • A description for the new xwalk has been added to EDU documentation site here
• If a new configuration variable was added:
  • The code is written such that the variable is optional (preferred), and this behavior was tested, OR
  • The code is written such that the variable is required, and a default value was added to edu_project_template, and this PR is flagged as breaking change (not for patch release)
  • A description for the new variable has been added to EDU documentation site here
• Reviewer confirms the grain of all tables are unchanged, OR any changes are expected, communicated, and this PR is flagged as a breaking change (not for patch release)