Add AWS doc to WAF fully qualified label naming definition

[saketh-bfs]

I added a reference to this page for label syntax https://docs.aws.amazon.com/waf/latest/developerguide/waf-rule-label-requirements.html

I needed to use this reference as I was using non rule group based rules and the examples in terraform provider docs werent sufficient. This doc defines the fully qualified label name concept that isnt complete in the terraform provider docs.

[github-actions]

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