Fix definition for bulk write update operation

[isabelatkinson]

The collation field was inadvertently left out of the definition for the update operation document. This PR adds it in and clarifies how the updateMods field should be populated (see this thread). collation is already present in the update/replace write models and is tested in this spec test.

Please complete the following before merging:

• Update changelog.
• Make sure there are generated JSON files from the YAML test files.
• Test changes in at least one language driver.
• Test these changes against all server versions and topologies (including standalone, replica set, sharded
  clusters, and serverless).