Document that ACL tests are run when creating and updating tailscale_acl resources.

[dsymonds]

Fixes #158.
Closes #161.

[dsymonds]

@clstokes @davidsbond @knyar

I've tested this manually, but not sure how easy or useful it is to add a unit test.

[knyar]

Thank you! I might be missing something, but what does this achieve if validation is only called immediately before attempting to set ACL contents with the same value? The same validation (including tests) will be applied while ACL content is being set, and an error should be returned if any of the tests fail.

[dsymonds]

That's a good point! For some reason I thought this wasn't happening, but just tested it manually now and the tests do indeed get exercised.

Okay, let me pare this right back to just the documentation update.

[dsymonds]

PTAL @knyar

[knyar]

I think the markdown files get generated based on the source code, so I suspect you'd actually need to make the change in resource_acl.go and run go generate.

[dsymonds]

Ah ha! I overlooked the very subtle "generated" comment at the top. Done properly now.

PTAL @knyar

[knyar]

Thank you!