revlog: move entry documentation alongside new related constants
Accessing individual index-entry element is usually done using integer directly.
This is presumably for "performance reasons". However as the index entry gain
more and more element it seems useful to get the option to use symbolic constant
to access item, for both clarify and versatility. We will probably keep using
integer for performance critical path, but less critical code may start using
them now that they are declared.
Differential Revision: https://phab.mercurial-scm.org/D10791
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