docs(material/table): add multiTemplateDataRows specification

[geromegrignon]

Purpose

Add a section to material/table documentation about how to retrieve the index of a row when using the multiTemplateDataRows directive. This information is only accessible as a comment into the row.ts file of the cdk.

        Context provided to the row cells when multiTemplateDataRows is true. This context is the same
        as CdkCellOutletRowContext except that the single index value is replaced by dataIndex and
        renderIndex.

Motivation

Having notifications over time about a related issue i gave an answer 2 years ago.

Issue #12793

[crisbeto]

LGTM with one minor comment.

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