[GHC] #14731: Document alignment & underlying size invariants for array types in GHC.Prim

[GHC]

#14731: Document alignment & underlying size invariants for array types in GHC.Prim
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        Reporter:  jberryman         |                Owner:  (none)
            Type:  task              |               Status:  new
        Priority:  normal            |            Milestone:
       Component:  Documentation     |              Version:  8.2.2
      Resolution:                    |             Keywords:
Operating System:  Unknown/Multiple  |         Architecture:
                                     |  Unknown/Multiple
 Type of failure:  None/Unknown      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:  #2917 #9806       |  Differential Rev(s):
       Wiki Page:                    |
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Comment (by jberryman):

 I updated the ticket to also mention the property that ByteArray have
 underlying size evenly divisible by word-size. It's not clear if that's
 something GHC wants to commit to as an invariant, e.g. I was just reading
 #11312

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/14731#comment:4>
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