Docs - added another common config property to tuningConfig

[petermarshallio]

I noticed that indexSpecForIntermediatePersists is a common configuration property across ingestion task types, so have added it alongside indexSpec on the main tuningConfig section.
Also updates the definition for indexSpec to closer mirror the definitions given in the tables for individual ingestion task types.

This PR has:

• been self-reviewed.
• been tested in a test Druid cluster.

[petermarshallio]

Would you mind doing a double-check that this is correct @sergioferragut ?
@techdocsmith another small one.

[techdocsmith]

LGTM

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