fix: Add information about valid export formats to help page (#1653)

Co-authored-by: finswimmer77@gmail.com <GiWahnsinn07!>

poetry/console/commands/export.py

@@ -11,7 +11,12 @@ class ExportCommand(Command):
     description = "Exports the lock file to alternative formats."
 
     options = [
-        option("format", "f", "Format to export to.", flag=False),
+        option(
+            "format",
+            "f",
+            "Format to export to. Currently, only requirements.txt is supported.",
+            flag=False,
+        ),
         option("output", "o", "The name of the output file.", flag=False),
         option("without-hashes", None, "Exclude hashes from the exported file."),
         option("dev", None, "Include development dependencies."),