Fix misleading `list-dids` example and clarify default `FILE` exclusion

Description

The current documentation for "Querying DIDs" contains an incorrect example that shows FILE DIDs being returned by default. In reality, rucio list-dids only returns collections (DATASET/CONTAINER) unless filtered.

The Error

The docs currently show:

$ rucio list-dids user.jdoe:*
# ... table shows FILE types here (This is incorrect behavior)

The Fix

1. Update the example: Remove the FILE entries from the default output table or update the command to include --filter type=FILE.
2. Add a Note: Explicitly state that files are excluded by default for scalability/performance reasons.

Context

This stems from the discussion in rucio/rucio issue rucio/rucio#8519.

AI Disclosure

In accordance with the Rucio AI Policy, I disclose that this issue was drafted with the assistance of an AI tool. I have reviewed the technical details (referencing issue rucio/rucio#8519 and the current documentation) and confirm that I fully understand the proposed changes and the discrepancy being reported.