arrgh, sniped by @Aasawari (happily with a code example) my pipeline is slightly different.
A pipeline would need to be added to the watch.
Something like:
[
{ "$match":
{ "operationType": "update",
"fullDocument.available": true,
"fullDocument.language": "english",
"updateDescription.updatedFields.location": { "$exists": true }
}
}
]
This would strictly match updates where the field location is updated along with the matching criteria for available and language.
If you want to also catch other scenarios like a new matching document being inserted the the pipeline would need further modification.