[Analytics] Document that GraphQL query inputs cannot exceed 25KB

We're imposing a limit on the sizes of GraphQL queries - that is,
the input data, and not the output. This commit documents that
change. I wasn't sure how best to convey that it isn't the response
that is being limited. Ideas for rephrasing are welcome.

Author: 46bit

content/analytics/graphql-api/limits/index.md

@@ -24,6 +24,7 @@ limits:
 
 These limits are applied to every query for every plan:
 
+* GraphQL inputs cannot be larger than **25KB**
 * A zone-scoped query can include up to **10 zones**
 * An account-scoped query can include only **1 account**