docs: add StaticQuery reference note (#7640)
docs: add StaticQuery reference note (#7640)

* Add StaticQuery reference note

* Shorten text

* docs: add reference to the issue about StaticQuery variables

Add a reference to #6545 to give more context to _why_ this won't currently work.

gatsbyjs/gatsby,ChristopherBiscardi/gatsby,ChristopherBiscardi/gatsby,ChristopherBiscardi/gatsby,mingaldrichgan/gatsby,gatsbyjs/gatsby,0x80/gatsby,gatsbyjs/gatsby,0x80/gatsby,0x80/gatsby,gatsbyjs/gatsby,mingaldrichgan/gatsby,gatsbyjs/gatsby,ChristopherBiscardi/gatsby,gatsbyjs/gatsby,mingaldrichgan/gatsby