Allow sqlite-utils upsert to infer --pk from existing table

docs/cli-reference.rst

@@ -321,6 +321,8 @@ See :ref:`cli_upsert`.
       incoming record has a primary key that matches an existing record the existing
       record will be updated.
 
+      If the table already exists and has a primary key, --pk can be omitted.
+
       Example:
 
           echo '[
@@ -330,7 +332,6 @@ See :ref:`cli_upsert`.
 
     Options:
       --pk TEXT                 Columns to use as the primary key, e.g. id
-                                [required]
       --flatten                 Flatten nested JSON objects, so {"a": {"b": 1}}
                                 becomes {"a_b": 1}
       --nl                      Expect newline-delimited JSON