Data plugin README (#78750)

* data readme

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docs/developer/plugin-list.asciidoc

@@ -48,7 +48,7 @@ NOTE:
 
 
 |{kib-repo}blob/{branch}/src/plugins/data/README.md[data]
-|data plugin provides common data access services.
+|The data plugin provides common data access services, such as search and query, for solutions and application developers.
 
 
 |{kib-repo}blob/{branch}/src/plugins/dev_tools/README.md[devTools]

src/plugins/data/README.md

@@ -1,9 +1,137 @@
 # data
 
-`data` plugin provides common data access services.
+The data plugin provides common data access services, such as `search` and `query`, for solutions and application developers.
 
-- `expressions` — run pipeline functions and render results.
-- `filter`
-- `index_patterns`
-- `query`
-- `search`: Elasticsearch API service and strategies
+## Autocomplete
+
+The autocomplete service provides suggestions for field names and values.
+
+It is wired into the `TopNavMenu` component, but can be used independently.
+
+### Fetch Query Suggestions
+
+The `getQuerySuggestions` function helps to construct a query. 
+KQL suggestion functions are registered in X-Pack, so this API does not return results in OSS.
+
+```.ts
+
+    // `inputValue` is the user input
+    const querySuggestions = await autocomplete.getQuerySuggestions({
+        language: 'kuery',
+        indexPatterns: [indexPattern],
+        query: inputValue,
+    });
+
+```
+
+### Fetch Value Suggestions
+
+The `getValueSuggestions` function returns suggestions for field values.
+This is helpful when you want to provide a user with options, for example when constructing a filter. 
+
+```.ts
+
+    // `inputValue` is the user input
+    const valueSuggestions = await autocomplete.getValueSuggestions({
+      indexPattern,
+      field,
+      query: inputValue,
+    });
+
+```
+
+## Field Formats
+
+Coming soon.
+
+## Index Patterns
+
+Coming soon.
+
+## Query
+
+The query service is responsible for managing the configuration of a search query (`QueryState`): filters, time range, query string, and settings such as the auto refresh behavior and saved queries.
+
+It contains sub-services for each of those configurations:
+ - `data.query.filterManager` - Manages the `filters` component of a `QueryState`. The global filter state (filters that are persisted between applications) are owned by this service.
+ - `data.query.timefilter` - Responsible for the time range filter and the auto refresh behavior settings.
+ - `data.query.queryString` - Responsible for the query string and query language settings.
+ - `data.query.savedQueries` - Responsible for persisting a `QueryState` into a `SavedObject`, so it can be restored and used by other applications.
+
+ Any changes to the `QueryState` are published on the `data.query.state$`, which is useful when wanting to persist global state or run a search upon data changes.
+
+ A simple use case is:
+
+ ```.ts
+ function searchOnChange(indexPattern: IndexPattern, aggConfigs: AggConfigs) {
+    data.query.state$.subscribe(() => {
+
+        // Constuct the query portion of the search request
+        const query = data.query.getEsQuery(indexPattern);
+
+        // Construct a request
+        const request = {
+            params: {
+                index: indexPattern.title,
+                body: {
+                    aggs: aggConfigs.toDsl(),
+                    query,
+                },
+            },
+        };
+
+        // Search with the `data.query` config
+        const search$ = data.search.search(request);
+
+        ...
+    });
+ }
+
+ ```
+
+## Search
+
+Provides access to Elasticsearch using the high-level `SearchSource` API or low-level `Search Strategies`.
+
+### SearchSource
+
+The `SearchSource` API is a convenient way to construct and run an Elasticsearch search query.
+
+```.tsx
+
+    const searchSource = await data.search.searchSource.create();
+    const searchResponse = await searchSource
+      .setParent(undefined)
+      .setField('index', indexPattern)
+      .setField('filter', filters)
+      .fetch();
+
+```
+
+### Low-level search
+
+#### Default Search Strategy
+
+One benefit of using the low-level search API, is partial response support in X-Pack, allowing for a better and more responsive user experience.
+In OSS only the final result is returned. 
+
+```.ts
+    import { isCompleteResponse } from '../plugins/data/public';
+
+    const search$ = data.search.search(request)
+        .subscribe({
+        next: (response) => {
+            if (isCompleteResponse(response)) {
+                // Final result
+                search$.unsubscribe();
+            } else {
+                // Partial result - you can update the UI, but data is still loading
+            }
+        },
+        error: (e: Error) => {
+            // Show customized toast notifications. 
+            // You may choose to handle errors differently if you prefer. 
+            data.search.showError(e);
+        },
+    });
+```