Add enum values to s3 checksum help text

The help text for --checksum-mode and --checksum-algorithm
did not document accepted enum values, making it unclear
what options are available without consulting external docs.

Update --checksum-mode to state ENABLED is the only valid
value. Update --checksum-algorithm to list valid values
(CRC64NVME, CRC32, SHA256, SHA1, CRC32C) and note the
default.

Fixes aws#10025

Author: adev-code

awscli/customizations/s3/subcommands.py

@@ -454,13 +454,17 @@
 
 CHECKSUM_MODE = {
         'name': 'checksum-mode', 'choices': ['ENABLED'],
-        'help_text': 'To retrieve the checksum, this mode must be enabled. If the object has a '
-                     'checksum, it will be verified.'
+        'help_text': 'To retrieve the checksum, this mode must be enabled. ENABLED is the only '
+                     'valid value. If the object has a checksum, it will be verified.'
 }
 
 CHECKSUM_ALGORITHM = {
         'name': 'checksum-algorithm', 'choices': ['CRC64NVME', 'CRC32', 'SHA256', 'SHA1', 'CRC32C'],
-        'help_text': 'Indicates the algorithm used to create the checksum for the object.'
+        'help_text': (
+            'Indicates the algorithm used to create the checksum for the object. '
+            'Valid values are: CRC64NVME | CRC32 | SHA256 | SHA1 | CRC32C. '
+            'Defaults to \'CRC64NVME\'.'
+        )
 }
 
 BUCKET_NAME_PREFIX = {