docs: add link to Load Balancer types doc

[modular-magician]

This PR adds a link to GCP documentation that explains the various Load Balancer types.

(The behavior each balancing_mode value represents is unclear; I'm hoping that linking to the documentation will change that! 🙂 )

Checklist

If this PR is for Terraform, I acknowledge that I have:

• Searched through the issue tracker for an open issue that this either resolves or contributes to, commented on it to claim it, and written "fixes {url}" or "part of {url}" in this PR description. If there were no relevant open issues, I opened one and commented that I would like to work on it (not necessary for very small changes).
• Generated Terraform, and ran make test and make lint to ensure it passes unit and linter tests.
• Ensured that all new fields I added that can be set by a user appear in at least one example (for generated resources) or third_party test (for handwritten resources or update tests).
• Ran relevant acceptance tests (If the acceptance tests do not yet pass or you are unable to run them, please let your reviewer know).
• Read the Release Notes Guide before writing my release note below.

Notes:

• I have not run tests, as this was a (seemingly minor) docs change.
• Likewise, I did not create a separate issue for the same reason.

Derived from GoogleCloudPlatform/magic-modules#6157