Fix issues with placeholder templating docs

[jpy-git]

Brief summary of the change made

Fixes #1996.
This PR clarifies the placeholder templating documentation.

1. Fixes docs referring to the wrong config section. Should refer to [sqlfluff:templater:placeholder]
2. Adds note on using quotes around custom param_regex.
3. Adds an example for using positional param style.
4. Adds note on enabling placeholder templating.

Are there any other side effects of this change that we should be aware of?

No, fixes docs to match code

Pull Request checklist

• Please confirm you have completed any of the necessary steps below.

• Included test cases to demonstrate any code changes, which may be one or more of the following:

  • .yml rule test cases in test/fixtures/rules/std_rule_cases.
  • .sql/.yml parser test cases in test/fixtures/dialects (note YML files can be auto generated with python test/generate_parse_fixture_yml.py or by running tox locally).
  • Full autofix test cases in test/fixtures/linter/autofix.
  • Other.

• Added appropriate documentation for the change.

• Created GitHub issues for any relevant followup/future enhancements if appropriate.

[jpy-git]

@barrywhart @tunetheweb

[tunetheweb]

LGTM though can't say I know the placeholder templater, but am assume you do :-)