docs: improve command line help

data_diff/__main__.py

@@ -124,7 +124,14 @@ def write_usage(self, prog: str, args: str = "", prefix: Optional[str] = None) -
 @click.argument("database2", required=False)
 @click.argument("table2", required=False)
 @click.option(
- "-k", "--key-columns", default=[], multiple=True, help="Names of primary key columns. Default='id'.", metavar="NAME"
+ "-k",
+ "--key-columns",
+ default=[],
+ multiple=True,
+ help="Names of primary key columns. Default='id'."
+ "Can be used more than once in the same command for composite key. "
+ "Example: -k col_a -k col_b",
+ metavar="NAME",
 )
 @click.option("-t", "--update-column", default=None, help="Name of updated_at/last_updated column", metavar="NAME")
 @click.option(