Mention Jumbo Frames for VPC MTU

[modular-magician]

Adjust description for VPC MTU to mention jumbo frames.
Currently, the field says the maximum is 1500 bytes which is not correct, but it's also not enforced. You can set it to 8896 bytes for jumbo frames and it will work, just the description says it's wrong.

The idea with this change is to convey the most important bits of information from here: https://cloud.google.com/vpc/docs/mtu

Since there's various cases to be covered I don't know what would be the best way to convey this in a short description, but here's my shot on that.

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). -> Very small change.
• Generated Terraform, and ran make test and make lint to ensure it passes unit and linter tests. -> No, first the link is broken, second that version of Ruby is too ancient to build properly. It's just a description change anyway, maybe a linter check would be nice but I can't get it working without Ruby. Or maybe I am just doing wrong, idk but it's just a description change.
• 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). -> There's no new field.
• Ran relevant acceptance tests (If the acceptance tests do not yet pass or you are unable to run them, please let your reviewer know). -> Cannot run.
• Read the Release Notes Guide before writing my release note below.

Release Note Template for Downstream PRs (will be copied)

compute: fixed wrong maximum limit description for possible VPC MTUs

Derived from GoogleCloudPlatform/magic-modules#7221