Identity lookup implementation for REST backend

docs/backends/rest.md

@@ -93,8 +93,8 @@ Encoding: JSON UTF-8
 ```
 
 #### Response Body
-`lookup.uid` contains the Matrix ID localpart of the user
-
+If a match was found:
+- `lookup.id.type` supported values: `localpart`, `mxid`
 ```
 {
   "lookup": {
@@ -108,6 +108,11 @@ Encoding: JSON UTF-8
 }
 ```
 
+If no match was found:
+```
+{}
+```
+
 #### Bulk
 Configured with `rest.endpoints.identity.bulk`
 
@@ -131,6 +136,8 @@ Encoding: JSON UTF-8
 ```
 
 #### Response Body
+For all entries where a match was found:
+- `lookup[].id.type` supported values: `localpart`, `mxid`
 ```
 {
   "lookup": [
@@ -153,3 +160,10 @@ Encoding: JSON UTF-8
   ]
 }
 ```
+
+If no match was found:
+```
+{
+  "lookup": []
+}
+```