[m-rev.] diff: restore --show-definitions documentation in usage message

[Julien Fischer]

Restore --show-definitions documentation in usage message.

compiler/options.m:
    As above; it was inadvertently deleted in commit bbf3da0fd.

Julien.

diff --git a/compiler/options.m b/compiler/options.m
index 147ce2d..de50c4a 100644
--- a/compiler/options.m
+++ b/compiler/options.m
@@ -4440,6 +4440,10 @@ options_help_aux_output -->
          "\tSet the maximum width of an error message line to <n> characters",
          "\t(unless a long single word forces the line over this limit).",
          "\tSpecifying --no-max-error-line-width removes the limit.",
+        "--show-definitions",
+        "\tWrite out a list of the types, insts, modes, predicates, functions",
+        "\ttypeclasses and instances defined in the module to",
+        "\t`<module>.defns'.",
          "--show-definition-line-counts",
          "\tWrite out a list of the predicates and functions defined in",
          "\tthe module, together with the names of the files containing them",