pseq: Update documentation for multi-chassis

Update the phosphor-power-sequencer documentation to define the support
required for multiple chassis systems.

The support described is currently only i

pseq: Update documentation for multi-chassis

Update the phosphor-power-sequencer documentation to define the support
required for multiple chassis systems.

The support described is currently only in the design stage. After this
review is complete, future commits will implement the multiple chassis
support.

The goal is for the phosphor-power-sequencer support to be largely
unchanged for single chassis systems.

This documentation also refers to planned enhancements to two other
applications:
* phosphor-chassis-state-manager (enhance multi-chassis support)
* phosphor-chassis-power (new application)

Change-Id: I10ed0ba3c73008ba297c8c7b47293eefc998a2ea
Signed-off-by: Shawn McCarney <shawnmm@us.ibm.com>

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