Updates the wording of several strings in one request, for one language. Strings not
included are left as they are.

• Every item in one request must use the same language (and branding, if you send it).
  Send one request per language.
• Each item is applied as Update a string would apply it:
  sending the wording the string already inherits removes your override.
• An id whose set (the part before the first dot) is not recognised rejects the request
  before anything is written. Only use ids returned by List strings.
• The update is not atomic. Sets are written one after another, so a failure part-way leaves
  the earlier sets saved. Re-sending the same request is safe.
• Send requests one at a time. Writes to a branding are queued, and a write still waiting
  after 10 seconds is refused (BOLT-7059).
• The request body must be under 10 MB.