Minor improvements to typecast section of manual (#14896)

* Minor improvements to typecast section of manual

* Clarification to casting w/ concrete types

* Added less ambiguous language

doc/manual.rst

@@ -3230,7 +3230,7 @@ has lots of advantages:
 Type conversions
 ----------------
 
-Syntactically a `type conversion` is like a procedure call, but a
+Syntactically a *type conversion* is like a procedure call, but a
 type name replaces the procedure name. A type conversion is always
 safe in the sense that a failure to convert a type to another
 results in an exception (if it cannot be determined statically).
@@ -3265,15 +3265,26 @@ conversions too are now *always unchecked*.
 
 Type casts
 ----------
-Example:
+
+*Type casts* are a crude mechanism to interpret the bit pattern of an expression
+as if it would be of another type. Type casts are only needed for low-level
+programming and are inherently unsafe.
 
 .. code-block:: nim
   cast[int](x)
+  
+The target type of a cast must be a concrete type, for instance, a target type
+that is a type class (which is non-concrete) would be invalid:
 
-Type casts are a crude mechanism to interpret the bit pattern of
-an expression as if it would be of another type. Type casts are
-only needed for low-level programming and are inherently unsafe.
-
+.. code-block:: nim
+  type Foo = int or float
+  var x = cast[Foo](1) # Error: cannot cast to a non concrete type: 'Foo'
+  
+Type casts should not be confused with *type conversions,* as mentioned in the
+prior section. Unlike type conversions, a type cast cannot change the underlying 
+bit pattern of the data being casted (aside from that the size of the target type
+may differ from the source type). Casting resembles *type punning* in other
+languages or C++'s ``reinterpret_cast`` and ``bit_cast`` features.
 
 The addr operator
 -----------------