Docs: Add argument validation to GraphQL Styleguide

What does this MR do?

This MR came out of this discussion !24027 (diffs, comment 289095528).

Sometimes GraphQL mutations and resolvers may accept a number of optional arguments but still want to validate that at least one of the optional arguments were given. This change adds to the GraphQL style guide a new section to do with validating arguments. It mentions that we can use the #ready? method for the situation of needing to validate across multiple arguments.

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