Added buffer variable documentation

[fivetran-avinash]

PR Overview

This PR will address the following Issue/Feature: [#112]

This PR will result in the following new package version:

No new package version, updates formally announced when jira calendar spine fixes are validated

Please provide the finalized CHANGELOG entry which details the relevant changes included in this PR:

CHANGELOG will not change, documentation for this update will appear in the next version of the dbt_jira package

PR Checklist

Basic Validation

Please acknowledge that you have successfully performed the following commands locally:

• [NA] dbt run –full-refresh && dbt test
• [NA] dbt run (if incremental models are present) && dbt test

Before marking this PR as "ready for review" the following have been applied:

• [NA] The appropriate issue has been linked, tagged, and properly assigned
• All necessary documentation and version upgrades have been applied
• [NA] docs were regenerated (unless this PR does not include any code or yml updates)
• BuildKite integration tests are passing
• [NA] Detailed validation steps have been provided below

Detailed Validation

Please share any and all of your validation steps:
Documentation update

If you had to summarize this PR in an emoji, which would it be?

⌨️

[fivetran-joemarkiewicz]

LGTM! No release needed, feel free to merge to main