revlog: improve documentation of the entry tuple
The code in revlog, and outside revlog directly use the index's entry tuple,
with direct integer indexing. This is a voluntary trade off to obtains better
performance from the Python code at the expense of the developers sanity.
Let's at least have a clear and central documentation about what this tuple is
about.
Differential Revision: https://phab.mercurial-scm.org/D10643
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