Fix docstrings of on_fit_start/end #12016

Author: daniellepintz

pytorch_lightning/trainer/callback_hook.py

@@ -117,7 +117,7 @@ def on_fit_start(self):
         .. deprecated:: v1.6
             `TrainerCallbackHookMixin.on_fit_start` was deprecated in v1.6 and will be removed in v1.8.
 
-        Called when the trainer initialization begins, model has not yet been set.
+        Called when fit begins.
         """
         rank_zero_deprecation(
             "`TrainerCallbackHookMixin.on_fit_start` was deprecated in v1.6 and will be removed in v1.8."
@@ -130,7 +130,7 @@ def on_fit_end(self):
         .. deprecated:: v1.6
             `TrainerCallbackHookMixin.on_fit_end` was deprecated in v1.6 and will be removed in v1.8.
 
-        Called when the trainer initialization begins, model has not yet been set.
+        Called when fit ends.
         """
         rank_zero_deprecation(
             "`TrainerCallbackHookMixin.on_fit_end` was deprecated in v1.6 and will be removed in v1.8."