[GHC] #8864: Document heap allocation functions in StgCmmMonad/StgCmmLayout

[GHC]

#8864: Document heap allocation functions in StgCmmMonad/StgCmmLayout
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        Reporter:  tibbe             |            Owner:
            Type:  feature request   |           Status:  new
        Priority:  normal            |        Milestone:
       Component:  Documentation     |          Version:  7.6.3
      Resolution:                    |         Keywords:
Operating System:  Unknown/Multiple  |     Architecture:  Unknown/Multiple
 Type of failure:  None/Unknown      |       Difficulty:  Unknown
       Test Case:                    |       Blocked By:
        Blocking:                    |  Related Tickets:
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Comment (by tibbe):

 That does help. Clarifying the invariants (e.g. when does a value returned
 by `getHpRelOffset` become invalid) would also help.

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/8864#comment:4>
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