add missing overfit_pct docs (#1204)

* add missing overfit_pct docs

* move arg to old position

* move arg docs

pytorch_lightning/trainer/__init__.py

@@ -549,8 +549,8 @@ Example::
 
 overfit_pct
 ^^^^^^^^^^^
-Uses this much data of all datasets.
-Useful for quickly debugging or trying to overfit on purpose
+Uses this much data of all datasets (training, validation, test).
+Useful for quickly debugging or trying to overfit on purpose.
 
 Example::
 
@@ -560,6 +560,19 @@ Example::
     # use only 1% of the train, test, val datasets
     trainer = Trainer(overfit_pct=0.01)
 
+    # equivalent:
+    trainer = Trainer(
+        train_percent_check=0.01,
+        val_percent_check=0.01,
+        test_percent_check=0.01
+    )
+
+See Also:
+    - `train_percent_check`_
+    - `val_percent_check`_
+    - `test_percent_check`_
+
+
 precision
 ^^^^^^^^^
 Full precision (32), half precision (16).

pytorch_lightning/trainer/trainer.py

@@ -163,6 +163,8 @@ class Trainer(
 
             progress_bar_refresh_rate: How often to refresh progress bar (in steps)
 
+            overfit_pct: How much of training-, validation-, and test dataset to check.
+
             track_grad_norm: -1 no tracking. Otherwise tracks that norm
 
             check_val_every_n_epoch: Check val every n train epochs.