fixup! Specify update semantics (DataBiosphere/azul#2884)

Clarify "If a (b) then c (d)" language.

#6 (comment)
#6 (comment)

docs/dcp2_system_design.rst

@@ -1204,8 +1204,8 @@ Rename or update a data file
    applies to the former.
 
    If the ``file_name`` property is the same as in the original and the
-   staging area has the `is_delta`` property set to ``true`` (``false``), the
-   ``sha1`` and ``sha256`` properties must (should) differ from their
+   staging area has the `is_delta`` property set to ``true`` (or ``false``),
+   the ``sha1`` and ``sha256`` properties must (or should) differ from their
    respective values in the orginal. [#]_ In other words, redundant updates
    are disallowed in delta staging areas, but discouraged otherwise.