[spi_host doc] Minor updates to Theory of Operation
[spi_host doc] Minor updates to Theory of Operation

This update highlights a number of behavioral details that were noticed when writing
the implementation Design Details section.

For example, this update documents
- The specific idle/delay behavior when updating configuration settings between commands
- The definition of stall events.

Signed-off-by: Martin Lueker-Boden <7e82c17fafb3b7b91b3c3396a2f21a060e5a6c3d@wdc.com>

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