Improve CLI date argument help text documentation (#59797)

The help text for --start-date and --end-date arguments only
documented the YYYY-MM-DD format, but the actual implementation
uses pendulum.parse() which accepts a much wider variety of formats.

This commit updates the help text to accurately document the
commonly used formats:
- YYYY-MM-DD (date only)
- YYYY-MM-DDTHH:MM:SS (datetime)
- YYYY-MM-DDTHH:MM:SSHH:MM (datetime with timezone, ISO 8601)

The help text also references pendulum.parse() to indicate that
additional formats are supported, improving clarity for users.

Fixes: Incomplete documentation of date format options

Author: Arunodoy18

airflow-core/src/airflow/cli/cli_config.py

@@ -167,8 +167,24 @@ def string_lower_type(val):
     default=None,
     action="append",
 )
-ARG_START_DATE = Arg(("-s", "--start-date"), help="Override start_date YYYY-MM-DD", type=parsedate)
-ARG_END_DATE = Arg(("-e", "--end-date"), help="Override end_date YYYY-MM-DD", type=parsedate)
+ARG_START_DATE = Arg(
+    ("-s", "--start-date"),
+    help=(
+        "Override start_date. Accepts multiple datetime formats including: "
+        "YYYY-MM-DD, YYYY-MM-DDTHH:MM:SS, YYYY-MM-DDTHH:MM:SS±HH:MM (ISO 8601), "
+        "and other formats supported by pendulum.parse()"
+    ),
+    type=parsedate,
+)
+ARG_END_DATE = Arg(
+    ("-e", "--end-date"),
+    help=(
+        "Override end_date. Accepts multiple datetime formats including: "
+        "YYYY-MM-DD, YYYY-MM-DDTHH:MM:SS, YYYY-MM-DDTHH:MM:SS±HH:MM (ISO 8601), "
+        "and other formats supported by pendulum.parse()"
+    ),
+    type=parsedate,
+)
 ARG_OUTPUT_PATH = Arg(
     (
         "-o",