Add documentation.

docs/assembly.rst

@@ -405,6 +405,16 @@ changes during the call, and thus references to local variables will be wrong.
         }
     }
 
+.. note::
+    If you access variables of a type that spans less than 256 bits
+    (for example ``uint64``, ``address``, ``bytes16`` or ``byte``),
+    you cannot make any assumptions about bits not part of the
+    encoding of the type. Especially, do not assume them to be zero.
+    To be safe, always clear the data properly before you use it
+    in a context where this is important:
+    ``uint32 x = f(); assembly { x := and(x, 0xffffffff) /* now use x */ }``
+    To clean signed types, you can use the ``signextend`` opcode.
+
 Labels
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