Grid docs: explain sorter aria-label generation

[rolfsmeds]

aria-label attributes are autogenerated for Grid column sorters based on the text content in the sorter, prefixed by "Sort by ", so e.g. a column with the header text "Surname" would get aria-label="Sort by Surname". Currently this does not work with column headers with Components.

These details should be mentioned in the Grid documentation (e.g. under Sorting section, or in a separate Accessibility section).