Query docs for custom operators don't mention they are custom operators

The [query docs for each custom operator](https://fsharp.github.io/fsharp-core-docs/reference/fsharp-linq-querybuilder.html) need a `remarks` section (or a mention in the summary) about how the custom opreator is used

At the moment there's no mention of the custom operator, and the usage text looks like a method call.

Two sides to this

* partly an FSharp.Core.xml problem (doc comment needs to mention this)

* partly an F#F problem (adjust usage text to show both method call and builder syntax)