Adjust docs referring to experimental queries to include details on new tagging system

Author: turbo

CONTRIBUTING.md

@@ -25,6 +25,7 @@ If you have an idea for a query that you would like to share with other CodeQL u
 
     Each language-specific directory contains further subdirectories that group queries based on their `@tags` or purpose.
     - Experimental queries and libraries are stored in the `experimental` subdirectory within each language-specific directory in the [CodeQL repository](https://github.com/github/codeql). For example, experimental Java queries and libraries are stored in `java/ql/src/experimental` and any corresponding tests in `java/ql/test/experimental`.
+    - Experimental queries need to include `experimental` in their `@tags`
     - The structure of an `experimental` subdirectory mirrors the structure of its parent directory.
     - Select or create an appropriate directory in `experimental` based on the existing directory structure of `experimental` or its parent directory.
 

docs/experimental.md

@@ -2,6 +2,8 @@
 
 In addition to [our supported queries and libraries](supported-queries.md), this repository also contains queries and libraries of a more experimental nature. Experimental queries and libraries can be improved incrementally and may eventually reach a sufficient maturity to be included in our supported queries and libraries.
 
+Experimental security queries are included in the `experimental` [CodeQL suite](https://docs.github.com/en/code-security/code-scanning/automatically-scanning-your-code-for-vulnerabilities-and-errors/configuring-code-scanning#using-queries-in-ql-packs). This suite is provided for testing purposes only, and should not be used in production code scanning workflows. 
+
 Experimental queries and libraries may not be actively maintained as the [supported](supported-queries.md) libraries evolve. They may also be changed in backwards-incompatible ways or may be removed entirely in the future without deprecation warnings.
 
 See [CONTRIBUTING.md](../CONTRIBUTING.md) for guidelines on submitting a new experimental query.