[T5 doc] Fix confusing documentation about `d_kv` (#21896)

* Confusing documentation in T5

* Fix onfusing documentation in T5 configuration file

src/transformers/models/t5/configuration_t5.py

@@ -48,8 +48,8 @@ class T5Config(PretrainedConfig):
         d_model (`int`, *optional*, defaults to 512):
             Size of the encoder layers and the pooler layer.
         d_kv (`int`, *optional*, defaults to 64):
-            Size of the key, query, value projections per attention head. `d_kv` has to be equal to `d_model //
-            num_heads`.
+            Size of the key, query, value projections per attention head. The `inner_dim` of the projection layer will
+            be defined as `num_heads * d_kv`.
         d_ff (`int`, *optional*, defaults to 2048):
             Size of the intermediate feed forward layer in each `T5Block`.
         num_layers (`int`, *optional*, defaults to 6):