docs: Fix documentation inconsistencies and incorrect YAML syntax

[infernus01]

Changes

This PR fixes documentation inconsistencies and incorrect YAML syntax across the Tekton Pruner docs.

1. TTL/History Limit Interaction

The actual behavior is that TTL and history limits are independent mechanisms - TTL will delete old runs even if you're under the history limit. History limits will delete excess runs even if TTL hasn't expired yet.

2. Selector YAML Syntax

The selector field in ResourceSpec is defined as []SelectorSpec (a slice), so the correct YAML format requires list syntax:

- selector:
  - matchLabels:
      key: value

Fixes #149

Submitter Checklist

As the author of this PR, please check off the items in this checklist:

• Has Docs if any changes are user facing, including updates to minimum requirements e.g. Kubernetes version bumps
• Has Tests included if any functionality added or changed
• pre-commit Passed
• Follows the commit message standard
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• Release notes contains the string "action required" if the change requires additional action from users switching to the new release

Release Notes

NONE

[infernus01]

/kind documentation

[anithapriyanatarajan]

/approve

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[anithapriyanatarajan]

/lgtm