doc/manual/types: add relevant links; replace word (#18925)

* doc/manual/types: add relevant links; replace word

The expressions describing nominal and structural type systems
didn't mention their actual names,
so the links should allow the uninitiated
to figure out what they're referring to
and learn more about the corresponding subjects.

Also, replaced "Another fundamental difference"
with "A fundamental difference",
since there was no previous fundamental difference
mentioned above that passage (not explicitly at least).

* remove trailing whitespace

doc/manual/types.rst

@@ -39,9 +39,11 @@ perhaps somewhat counterintuitively, often significantly simplify them.
 Describing Julia in the lingo of `type
 systems <https://en.wikipedia.org/wiki/Type_system>`_, it is: dynamic,
 nominative and parametric. Generic types can be parameterized,
-and the hierarchical relationships
-between types are explicitly declared, rather than implied by compatible
-structure. One particularly distinctive feature of Julia's type system
+and the hierarchical relationships between types are `explicitly
+declared <https://en.wikipedia.org/wiki/Nominal_type_system>`_,
+rather than `implied by compatible structure
+<https://en.wikipedia.org/wiki/Structural_type_system>`_.
+One particularly distinctive feature of Julia's type system
 is that concrete types may not subtype each other: all concrete types
 are final and may only have abstract types as their supertypes. While
 this might at first seem unduly restrictive, it has many beneficial
@@ -324,7 +326,7 @@ cannot be declared to be any smaller than eight bits.
 The types :obj:`Bool`, :class:`Int8` and :class:`UInt8` all have identical
 representations: they are eight-bit chunks of memory. Since Julia's type
 system is nominative, however, they are not interchangeable despite
-having identical structure. Another fundamental difference between them
+having identical structure. A fundamental difference between them
 is that they have different supertypes: :obj:`Bool`'s direct supertype is
 :class:`Integer`, :class:`Int8`'s is :obj:`Signed`, and :class:`UInt8`'s is :obj:`Unsigned`.
 All other differences between :obj:`Bool`, :class:`Int8`, and :class:`UInt8` are