[CARBONDATA-4333][Doc] Update the declaration of supported String data types

[tangchuan92]

Why is this PR needed? CHAR and VARCHAR as String data types are no longer supported in Carbon. They should be deleted from doc's desc.

What changes were proposed in this PR? CHAR and VARCHAR stop appearing as two String data types in doc.

Does this PR introduce any user interface change? No

Is any new testcase added? No

[CarbonDataQA2]

Can one of the admins verify this patch?

[marchpure]

LGTM