docs(viewer): document full table search semantics with a screenshot

The "Full Table Search" section said only that a search box exists. It now
states the behaviour that users actually hit:

- the search covers every column at once, character columns by case sensitive
  CONTAINS and numeric columns by exact match
- the Numeric toggle switches to exact numeric matching
- an applied filter scopes the search
- results are capped by DC_MAXOBS_WEBVIEW (linked to the options page), and the
  row count next to the table name reports the match count
- a search with no matches shows the "No data found with given conditions"
  panel

Adds a screenshot of the search in action (docs/img/full-table-search.png),
captured from a running instance via the Cypress suite in the dc repo.

docs/dcu-tableviewer.md

@@ -16,6 +16,16 @@ Row and Column level security can also be applied in VIEW mode, as can additiona
 
 A single search box can be used to make a full table search on any character or numeric value, using this [macro](https://core.sasjs.io/mp__searchdata_8sas.html).
 
+The search covers **every** column of the table at once, so you do not need to know which column holds the value you are looking for:
+
+- **Character search** (the default) is a case sensitive CONTAINS match against every character column, so `smith` finds `Smithson` and `Goldsmith`, but not `Smith`.
+- **Numeric search** (tick the Numeric box) is an exact match against every numeric column, so `42` will not find `420` or `4210`.
+- If a filter is applied, the search runs within the filtered rows rather than the whole table.
+- The result set is capped by the [`DC_MAXOBS_WEBVIEW`](/dcc-options/#dc_maxobs_webview) option (500 rows by default). The row count shown next to the table name is the number of rows the search matched, which can be higher than the number of rows displayed.
+- A search that matches nothing shows a "No data found with given conditions" panel rather than an empty grid.
+
+![Full table search](img/full-table-search.png)
+
 <iframe width="560" height="315" src="https://www.youtube.com/embed/i27w-xq85WQ" title="YouTube video player" frameborder="0" allow="accelerometer; autoplay; clipboard-write; encrypted-media; gyroscope; picture-in-picture" allowfullscreen></iframe>
 
 ## Options