[ng] refactor: support comments only for VarDef, not for ItemDef too

[ng] refactor: support comments only for VarDef, not for ItemDef too

Comments are actually handled and used for variable definitions only,
so no point in handling it also for rule definitions.  And this will
simplify our next step where we change how variable definitions (and
related comments) are stored and displayed.

* lib/Automake/ItemDef.pm (new): Don't take a $comment argument
anymore, and don't set a $self->{comment} field in the returned
object.
(comment): Remove this accessor method.
(Pod documentation): Adjusted.
* lib/Automake/Rule.pm (define): Don't pass a $comment argument
to the Automake::RuleDef constructor anymore.  No real loss here,
since we were unconditionally passing an empty comment anyway.
* lib/Automake/RuleDef.pm (new): Don't take a $comment argument
anymore, and don't pass it to the Automake::ItemDef constructor.
(Pod documentation): Adjusted.
* lib/Automake/VarDef.pm (new): Directly store the $comment
argument in the $self->{comment} field.  Don't pass the
Automake::ItemDef constructor anymore.
(comment): New accessor method (it is no more inherited from
Automake::ItemDef anymore).

Signed-off-by: Stefano Lattarini <stefano.lattarini@gmail.com>