linkage: update --test documentation

Adjust the wording to be in sync with the changes made in #424.

Library/Homebrew/dev-cmd/linkage.rb

@@ -7,8 +7,9 @@
 # formulae.
 #
 # Options:
-#  --test      - testing version: only display broken libs; exit non-zero if any
-#                breakage was found.
+#  --test      - testing version: only display broken libraries and possibly
+#                undeclared dependencies; exit non-zero if any breakage was
+#                found (i.e. any of these yield a nonempty result).
 #  --reverse   - For each dylib the keg references, print the dylib followed by the
 #                binaries which link to it.