Document max_fields_limit_exceeded

[CaroFG]

Some users have encountered a max_fields_limit_exceeded erros, and they usually think it's related to the maximum number of attributes per document, but it's not.

According to @Kerollmops:

this issue is related to the total number of field over all the documents. Most of the time it is due to people storing user names in a nested object. The engine understand those as fields. There is a maximum of 64k different fields.
{ "users": { "kero": 1, "tamo": 2, ... } }.

See open issue.

It's a current engine limitation, so I thought it should be documented.