Unclear override semantics for `power_level_content_override`

From [the spec](https://matrix.org/docs/spec/client_server/r0.5.0#post-matrix-client-r0-createroom):

> power_level_content_override: The power level content to override in the default power level event. This object is __applied on top of__ the generated m.room.power_levels event content prior to it being sent to the room. Defaults to overriding nothing.

It's unclear what the exact override semantics are here. Is the entire generated `m.room.power_levels` content discarded and replaced with the object specified here (ie. potentially removing keys)? Are they deep-merged? If yes, what are the exact merge semantics?

Start with the linked Matrix Client-Server r0.5.0 create-room specification section and the definition of power_level_content_override. Determine whether the override replaces the generated content or is merged, including exact handling of nested keys. Done means the specification clearly documents the chosen semantics.

Written by the indexing model from the issue text.