Clarify how the "hidden" concept is computed.
Clarify how the "hidden" concept is computed.

* subversion/libsvn_wc/entries.c:
  (svn_wc__entry_is_hidden): expand the code. add comments. add an
    assertion to demonstrate the restricted schedule values.


git-svn-id: f8a4e5e023278da1e04e203c7fe051e3c4285d88@941560 13f79535-47bb-0310-9956-ffa450edef68

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