Explicitly refer to "Single Field Matches" for positional "$ (update)" operator

Description

As of DOCS-3276, https://docs.mongodb.com/manual/reference/operator/update/positional/ provides instructions for matching multiple fields within an array for a positional update, using elemMatch.

However, given the caveats with matching multiple fields (SERVER-14662), I think "the positional $ operator acts as a placeholder for the first element that matches the query document" language is leading to confusion.

Would it be possible to reference something like "Update Embedded Documents Using a Single Field Match" in the Definition or Behavior section, as a prompt to direct readers to the "Update Embedded Documents Using Multiple Field Matches" section at the end of the page?

By way of example, this is explicit in the "$ (projection)" documentation at https://docs.mongodb.com/manual/reference/operator/projection/positional/#array-field-limitations, with the following language: "The query document should only contain a single condition on the array field being projected. Multiple conditions may override each other internally and lead to undefined behavior."

Thank you in advance!

Scope of changes

• Update and backport as needed