Allow underscores in nested object fields (#31896)

We've been meaning to do this for a while but haven't gotten around it to it.

Seen in Discord at https://discord.com/channels/1019350475847499849/1312480232548339752/1312480232548339752

If this looks good, I'll work with @atrakh on making sure the dashboard can accommodate these fields.

GitOrigin-RevId: 2183da4c0f721a9be2f8a841dd843932d37c517f

docs/production/state/limits.mdx

@@ -51,6 +51,8 @@ Applied per document and to any nested `Object` unless stated otherwise.
 
 - Field names must only contain non-control alphanumeric ASCII characters and
   underscores and must start with an alphabetic character or underscore.
+- Documents cannot contain top-level fields that start with an underscore, other
+  than the system-provided `_id` and `_creationTime` fields.
 - Strings must be valid Unicode sequences with no unpaired surrogates.
 
 ## Functions