update docs of length_penalty (#17022)

src/transformers/generation_utils.py

@@ -950,9 +950,9 @@ class GenerationMixin:
             eos_token_id (`int`, *optional*):
                 The id of the *end-of-sequence* token.
             length_penalty (`float`, *optional*, defaults to 1.0):
-                Exponential penalty to the length. 1.0 means no penalty. Set to values < 1.0 in order to encourage the
-                model to generate shorter sequences, to a value > 1.0 in order to encourage the model to produce longer
-                sequences.
+                 Exponential penalty to the length. 1.0 means that the beam score is penalized by the sequence length. 0.0 means no penalty. Set to values < 0.0 in order to encourage the
+                 model to generate longer sequences, to a value > 0.0 in order to encourage the model to produce shorter
+                 sequences.
             no_repeat_ngram_size (`int`, *optional*, defaults to 0):
                 If set to int > 0, all ngrams of that size can only occur once.
             encoder_no_repeat_ngram_size (`int`, *optional*, defaults to 0):