Clarify documentation for the `tools` field in the `init` action

[sidshank]

The CodeQL init action YML file has a field called tools. The field is optional, but when specified, should reference a URL to the CodeQL tools:

https://github.com/github/codeql-action/blob/40542d38bc4936c2ca7da883aeb050a6081b4b31/init/action.yml#L5

We need clear documentation for our customers on how they might specify the tools field to point to a specific version of CodeQL. For example, if a new version of CodeQL breaks their workflow for some reason, they might want to downgrade to the last working CodeQL version until the issue is resolved.

Specifically, it would be nice to have documentation on how users can derive / construct the URL that needs to be specified for the tools field.

[brendandburns]

fwiw, making this the complete URL (e.g. https://github.com/github/codeql-action/releases/download/codeql-bundle-v2.13.4/codeql-bundle-linux64.tar.gz) works in my setup.