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

[dsyme]

The query docs for each custom operator 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)

[dsyme]

@Krzysztof-Cieslak presumably these kind of fixes are also necessary for Saturn docs