Clarify `physical/logical` representation in Table Schema

[roll]

Overview

This paragraph - https://datapackage.org/specifications/table-schema/#physical-and-logical-representation

I think physical term might be confusing (see #621) as it seems to be really meaning lexical or textual while logical sounds easy to understand in my opinion while it might still need to be brainstormed

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Subissues:

• https://github.com/frictionlessdata/specs/issues/621