Add help output for --chunked (#1174)

Author: BoboTiG

httpie/cli/definition.py

@@ -711,9 +711,11 @@ def __iter__(self):
     '--chunked',
     default=False,
     action='store_true',
-    help="""
+    help='''
+    Enable streaming via chunked transfer encoding.
+    The Transfer-Encoding header is set to chunked.
 
-    """
+    '''
 )
 
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