docs: add reference for label_flag and label_setting rules

[abishekgiri]

What does this PR do?

This PR adds missing reference documentation for the label_flag and label_setting
rules in site/en/extending/config.md.

These rules are currently mentioned in the documentation, but do not have a
clear description of their purpose, attributes, or usage.

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What’s included?

• Expanded documentation in the Label-typed build settings section
• Clear explanations of:
  • label_flag
  • label_setting
• A description of how these rules are used for configuration and transitions
• A list of supported attributes:
  • name
  • build_setting_default
  • visibility
• A minimal example showing how to define and reference them

---

Why is this useful?

Users encountering label_flag and label_setting currently need to inspect
rule definitions or source code to understand how they work.

This change provides a concise reference-style explanation consistent with
other build setting documentation and improves discoverability for users
extending Bazel configuration.

---

Verification

• Manually reviewed the updated documentation for clarity and consistency
  with existing sections
• Verified that examples follow current Bazel syntax and documentation style

---

Fixes #16128

[gemini-code-assist]

Code Review

This pull request adds valuable documentation for the label_flag and label_setting rules, which was previously missing. The added content, including the example for label_flag, is clear and helpful. I have one suggestion to clarify the distinction between label_flag and label_setting regarding command-line configuration to prevent potential user confusion.

[gemini-code-assist]

This sentence is slightly misleading. While both label_flag and label_setting are label-typed build settings, only label_flag can be configured on the command line. label_setting is meant for internal configuration, for example via transitions. It would be clearer to explicitly state this difference.

Suggested change

	These rules serve as label-typed build settings used for transitions and command-line configuration. They function as aliases that can be redirected by the configuration.
	These rules are label-typed build settings. `label_flag` can be set on the command line, while `label_setting` is for internal configuration (e.g. via transitions). Both function as aliases that can be redirected by the configuration.