mpr/mps: Add comment explaining state transition

When we can't load a request due to a shortage of chains, we complete
the command's cm. However, to avoid an assert in mp?_complete_command,
we transition its state to INQUEUE. This transition is legitimate
because this is the only error path that terminates a cm before it's
enqueued and the only other alternative would be an additional transient
state that would add complexity w/o adding value. Add a comment
explainging all this because otherwise the transition can look a bit
weird.

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