[docs]: document private log limits

[critesjosh]

Note in docs that private logs to have at most 8 fields.

• For notes: 1 for storage slot, 1 for note type id, and 6 for custom fields.
• For events: 1 for storage slot, 1 for event type id, 1 for address tag, and 5 for custom fields.
  This is to make it more difficult to figure out what a tx is about when all the logs are the same length. In the current codebase, the max number of custom fields is 4 for note, and 5 for event. We can adjust the limit to allow more custom fields if necessary.

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