Chart: Add more clear docs for setting pod_template_file.yaml

[kaxil]

The values.yaml didn't have any docs on it.

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^ Add meaningful description above

Read the Pull Request Guidelines for more information.
In case of fundamental code change, 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 UPDATING.md.

[github-actions]

The PR is likely OK to be merged with just subset of tests for default Python and Database versions without running the full matrix of tests, because it does not modify the core of Airflow. If the committers decide that the full tests matrix is needed, they will add the label 'full tests needed'. Then you should rebase to the latest main or amend the last commit of the PR, and push it with --force-with-lease.

[kaxil]

Ran locally:

#################### Documentation will be built for 1 package(s) with 6 parallel jobs ####################
1. helm-chart
#################### Cleaning documentation files ####################
helm-chart                                                  : Cleaning files
#################### Scheduling documentation to build ####################
helm-chart                                                  : Scheduling documentation to build
#################### Running docs building ####################

helm-chart                                                  : Building documentation
helm-chart                                                  : Running sphinx. The output is hidden until an error occurs.
helm-chart                                                  : Finished docs building successfully
#################### Scheduling spell checking of documentation ####################
helm-chart                                                  : Scheduling spellchecking
#################### Running spell checking of documentation ####################

helm-chart                                                  : Checking spelling started
helm-chart                                                  : Finished spell-checking successfully
helm-chart                                                  : Checking spelling completed
Documentation build is successful