Merge pull request #3176 from zielmicha/ofoperator

Improve discoverability of 'of' operator

doc/manual/types.txt

@@ -588,7 +588,8 @@ can also be defined with indentation instead of ``[]``:
 
 Objects provide many features that tuples do not. Object provide inheritance
 and information hiding. Objects have access to their type at runtime, so that
-the ``of`` operator can be used to determine the object's type.
+the ``of`` operator can be used to determine the object's type. The ``of`` operator
+is similar to the ``instanceof`` operator in Java.
 
 .. code-block:: nim
   type

lib/system.nim

@@ -487,6 +487,7 @@ type
     ## See the full `exception hierarchy`_.
   ObjectConversionError* = object of Exception ## \
     ## Raised if an object is converted to an incompatible object type.
+    ## You can use ``of`` operator to check if conversion will succeed.
     ##
     ## See the full `exception hierarchy`_.
   FloatingPointError* = object of Exception ## \