Covering index if all requested columns exists in an index, not all rows.

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1. What is the URL/path of the document related to this issue?
   https://docs.pingcap.com/tidb/stable/explain-indexes

2. How would you like to improve it?
   From

TiDB supports the covering index optimization. If all rows can be retrieved from an index, TiDB will skip the second step that is usually required in an IndexLookup. Consider the following two examples:

to

TiDB supports the covering index optimization. If all requested columns can be retrieved from an index, TiDB will skip the second step that is usually required in an IndexLookup. Consider the following two examples:

Since a covering index means that all columns in the index (including possible Primary Key columns added implicitly, to find the full table row), it should be columns and not rows, perhaps with a bit more explaining why this is a very good optimization for using indexes.

Open the stable explain-indexes documentation page named in the issue and locate the covering index explanation. Update the wording from “all rows” to “all requested columns,” clarify the optimization as requested, and verify that the surrounding IndexLookup examples still match the explanation.