[SPARK-35311][SS][UI][DOCS] Structured Streaming Web UI state informa…

…tion documentation

### What changes were proposed in this pull request?
In this PR I'm adding Structured Streaming Web UI state information documentation.

### Why are the changes needed?
Missing documentation.

### Does this PR introduce _any_ user-facing change?
No.

### How was this patch tested?
```
cd docs/
SKIP_API=1 bundle exec jekyll build
```
Manual webpage check.

Closes #32433 from gaborgsomogyi/SPARK-35311.

Authored-by: Gabor Somogyi <gabor.g.somogyi@gmail.com>
Signed-off-by: Jungtaek Lim <kabhwan.opensource@gmail.com>

docs/web-ui.md

@@ -415,6 +415,7 @@ click a "run id" in the tables.
 
 <p style="text-align: center;">
   <img src="img/webui-structured-streaming-detail.png" title="Structured Streaming Query Statistics" alt="Structured Streaming Query Statistics">
+  <img src="img/webui-structured-streaming-detail2.png">
 </p>
 
 The statistics page displays some useful metrics for insight into the status of your streaming 
@@ -431,6 +432,11 @@ The tracked operations are listed as follows.
     * latestOffset & getOffset: Time taken to query the maximum available offset for this source.
     * queryPlanning: Time taken to generates the execution plan.
     * walCommit: Time taken to write the offsets to the metadata log.
+* **Global Watermark Gap.** The gap between batch timestamp and global watermark for the batch.
+* **Aggregated Number Of Total State Rows.** The aggregated number of total state rows.
+* **Aggregated Number Of Updated State Rows.** The aggregated number of updated state rows.
+* **Aggregated State Memory Used In Bytes.** The aggregated state memory used in bytes.
+* **Aggregated Number Of State Rows Dropped By Watermark.** The aggregated number of state rows dropped by watermark.
 
 As an early-release version, the statistics page is still under development and will be improved in
 future releases.