InfluxDB 3 Ent: specifying `--plugin-dir` assigns the process mode

If you specify `--plugin-dir`, InfluxDB 3 automatically assigns the process mode to the node.

The documentation does state use the --plugin-dir flag to start the processing engine, but adding clarity on the use of mode along with this will be helpful.

```+---------------+-----------------+--------------------------------------+---------------------------+------------+---------+-------------------------+
| node_id | node_catalog_id | instance_id | mode | core_count | state | updated_at |
+---------------+-----------------+--------------------------------------+---------------------------+------------+---------+-------------------------+
| v3test1b | 0 | xxx | [ingest, process] | 2 | running | 2025-05-21T00:34:00.373 |```

[Slack Message](https://influxdata.slack.com/archives/C084G9LR2HL/p1747835485875319?thread_ts=1747835469.374069&cid=C084G9LR2HL)

In the docs-v2 repository, locate the InfluxDB 3 Enterprise documentation for the --plugin-dir flag and read the surrounding guidance on starting the processing engine. Update the documentation to explain that specifying --plugin-dir assigns process mode to the node, and verify the resulting wording is clear alongside the existing mode information.

Written by the indexing model from the issue text.