runtime(doc): Improve the documentation for Vim9 classes (#13828)

runtime(doc): Improve the documentation for Vim9 classes (#13828)

* Emend textual typos
* Emend syntactic errors in examples
* Acknowledge no support for abstract static methods
* Acknowledge the non-ubiquity of instance qualification

"This" was never allowed in method declarations, e.g.:
    class A
        def this.M()
        enddef
    endclass

and, since patch 9.0.2167, "this" can no longer be used in field
declarations, e.g.:
    class B
        var this.f: string
    endclass

* Recognise abstract child classes
* Reword an ambiguous turn of phrase

Signed-off-by: Aliaksei Budavei <0x000c70@gmail.com>
Signed-off-by: Ernie Rael <errael@raelity.com>
Signed-off-by: Yegappan Lakshmanan <yegappan@yahoo.com>
Signed-off-by: Christian Brabandt <cb@256bit.org>