all: remove rwshared keyword, make its semantics default for shared (#5710)

doc/upcoming.md

@@ -35,13 +35,20 @@ atomic d := ...
 - `c` can be passed to coroutines an accessed
   *concurrently*.<sup>2</sup> In order to avoid data races it has to
   be locked before access can occur and unlocked to allow access to
-  other coroutines. This is done by the following block structure:
+  other coroutines. This is done by one the following block structures:
   ```v
   lock c {
       // read, modify, write c
       ...
   }
   ```
+  
+  ```v
+  rlock c {
+      // read c
+      ...
+  }
+  ```
   Several variables may be specified: `lock x, y, z { ... }`.
   They are unlocked in the opposite order.
 - `d` can be passed to coroutines and accessed *concurrently*,