Add help text for agent selector on environment settings page

Proposal

On the Environment settings page, we have an agent selector to set up the Kubernetes integration with the environment. This selector is populated with the list of agents that have been granted access to the current project or group via the user_access keyword in the agent configuration file.

While we do mention this in the docs, it will be helpful to highlight it on the UI. The proposal is to add a help text to the agent selection dropdown with these details.

Before	After

Copy:

Select an agent with Kubernetes access to the project or group. How do I grant Kubernetes access?

The link points to: https://docs.gitlab.com/ee/user/clusters/agent/user_access.html

Implementation guide

Update the environment_form.vue component. Use GlFormText to include the help text. Use helpPagePath helper to add a link to the documentation.

Update corresponding tests in environment_form_spec.js