SQL Expressions Documentation: Document MySQL Keywords up front

@slim-bean said to us on Grafana labs internal Slack: > weird, would you expect `plan` to be a keyword? > yeah seems like it is, i get a syntax error if i have a column named plan and try to use it, if i rename it, works fine I think it could be nice to our users if we: 1. Acknowledge the list of MySQL keywords in our docs, so they can be warned in advance of this failure mode, and can quickly confirm the problem is what they think it is, and 2. Present them with ways to workaround this problem (maybe just linking onwards to relevant documentation). eg. using the `label_replace` Prom function.