Help: Document Swift_DEPENDENCIES_FILE property defaults

Copy wording from the release notes.

Help/prop_sf/Swift_DEPENDENCIES_FILE.rst

@@ -2,4 +2,4 @@ Swift_DEPENDENCIES_FILE
 -----------------------
 
 This property sets the path for the Swift dependency file (swiftdeps) for the
-source.
+source.  If one is not specified, it will default to ``<OBJECT>.swiftdeps``.

Help/prop_tgt/Swift_DEPENDENCIES_FILE.rst

@@ -2,4 +2,4 @@ Swift_DEPENDENCIES_FILE
 -----------------------
 
 This property sets the path for the Swift dependency file (swiftdep) for the
-target.
+target.  If one is not specified, it will default to ``<TARGET>.swiftdeps``.