docs: note multi-select and pre-selection in ES|QL data source browser (#6188)

## Summary

Follow-up to #6082, which addressed parent issue #5701 but didn't
explicitly close the two sibling issues #6068 and #6072. This PR adds
the two behavioral details from #6072 that weren't covered:

- the data source browser supports **multi-select** (add/remove several
sources in one session)
- sources already present in the query appear **pre-selected**

These are the only details from #6068 and #6072 worth documenting; other
behaviors (replace/deselect for the fields browser, no-preselection,
formatting-preserving inserts) are either self-evident from the UI or
already implied by the existing wording.

## Resolves

Closes #6068
Closes #6072

Made with [Cursor](https://cursor.com)

Author: florent-leborgne

explore-analyze/discover/try-esql.md

@@ -98,7 +98,7 @@ serverless: ga
 
 When you write a query, the {{esql}} editor includes two interactive browsers that help you find available data sources and field names:
 
-- **Data source browser**: lists the data sources of the following types that you can query: **Alias**, **Index**, **Integration**, **Lookup Index**, **Stream**, and **Timeseries**. Selections are inserted into the `FROM` or `TS` command and existing sources stay preserved. When the query starts with `TS`, only time series data sources are listed.
+- **Data source browser**: lists the data sources of the following types that you can query: **Alias**, **Index**, **Integration**, **Lookup Index**, **Stream**, and **Timeseries**. The browser supports multi-select: you can add or remove several sources in one session, and sources already present in your query appear pre-selected. Selections are inserted into the `FROM` or `TS` command and existing sources stay preserved. When the query starts with `TS`, only time series data sources are listed.
 - **Fields browser**: lists fields for the data sources currently in your query and lets you insert one field at a time at the cursor position.
 
 :::{note}