feat(DocHtmlFormat): Add `displayrowcount` option for array attributes (refs #4851)

`displayrowcount=n` accepts an integer `n` that specifies when the
array's row count is displayed:

    n <= -1 never display array's row count
    n = 0   always display array's row count
    n > 0   display array's row count if row count > n

Default value is "10".