docs: improve description of task_instance_key_str

[kevinhongzl]

closes: #51315

This PR improves the description of the task_instance_key_str template variable by removing the word "unique" for accuracy and adding some details.

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^ Add meaningful description above Read the Pull Request Guidelines for more information. In case of fundamental code changes, an Airflow Improvement Proposal (AIP) is needed. In case of a new dependency, check compliance with the ASF 3rd Party License Policy. In case of backwards incompatible changes please leave a note in a newsfragment file, named {pr_number}.significant.rst or {issue_number}.significant.rst, in airflow-core/newsfragments.