docs: Update datafusion-cli doc for top-memory-consumers config (apache#20390)

## Which issue does this PR close?
- Closes apache#20389.

## Rationale for this change
If both `--memory-limit` or `--mem-pool-type` configs are not set in
`datafusion-cli` command, `unbounded` memory pool is being used and so
`unbounded` memory pool is not tracked pool so `--top-memory-consumers >
0` is valid as long as one of the runtime configs: memory_limit or
memory_pool is set by using `greedy` or `fair` memory pool (which can be
tracked by `TrackConsumersPool`) so this doc can be useful for the
end-user to use either `greedy` or `fair` memory pool when
`--top-memory-consumers > 0 `.

## What changes are included in this PR?
Updating `datafusion-cli` doc for `top-memory-consumers` config.

## Are these changes tested?
N/A due to updating the `datafusion-cli` doc.

## Are there any user-facing changes?
Yes, updating `datafusion-cli` doc.

Co-authored-by: Jonah Gao <jonahgao@msn.com>

Author: erenavsarogullari

docs/source/user-guide/cli/usage.md

@@ -58,7 +58,9 @@ OPTIONS:
             Specify the memory pool type 'greedy' or 'fair', default to 'greedy'
 
         --top-memory-consumers <TOP_MEMORY_CONSUMERS>
-            The number of top memory consumers to display when query fails due to memory exhaustion. To disable memory consumer tracking, set this value to 0 [default: 3]
+            The number of top memory consumers to display when query fails due to memory exhaustion.
+            To disable memory consumer tracking, set this value to 0 [default: 3].
+            Please set one of the runtime configs: '--memory-limit' or '--mem-pool-type' to see 'top-memory-consumers' result when memory is exhausted.
 
     -d, --disk-limit <DISK_LIMIT>
             Available disk space for spilling queries (e.g. '10g'), default to None (uses DataFusion's default value of '100g')