refine the doc about calldata

Author: haoyang9804

docs/contracts/visibility-and-getters.rst

@@ -221,4 +221,4 @@ to select individual struct members or provide a key for the mapping:
         a = data[arg1][arg2][arg3].a;
         b = data[arg1][arg2][arg3].b;
         e = data[arg1][arg2][arg3].e;
-    }
+    }

docs/types/reference-types.rst

@@ -34,6 +34,14 @@ non-persistent area where function arguments are stored, and behaves mostly like
     data location can also be returned from functions, but it is not possible to
     allocate such types.
 
+.. note::
+    Arrays and structs with ``calldata`` location declared in a function body
+    or as its return parameters must be assigned before being used or returned.
+    There are certain cases in which non-trivial control flow is used and the compiler
+    can't properly detect the initialization.
+    A common workaround in such cases is to assign the affected variable to itself before
+    the correct initialization takes place.
+
 .. note::
     Prior to version 0.6.9 data location for reference-type arguments was limited to
     ``calldata`` in external functions, ``memory`` in public functions and either