[Doc] Update docstrings

Docstrings are improved for the `__getitem__` magic method and the
`preprocess` method for the `PyCytoData` class.

PyCytoData/data.py

@@ -189,6 +189,7 @@ def preprocess(self,
         3. Gate for intact cells.
         4. Gate for live cells.
         5. Gate for anomalies using center, offset, and residual channels. 
+        6. Bead normalization.
 
         :param gate_debris_removal: Whether to gate to remove debris, defaults to True.
         :type gate_debris_removal: bool
@@ -519,11 +520,11 @@ def __getitem__(self, items: Union[slice, List[int], np.ndarray, Tuple[Union[sli
         
         A few deviations from the numpy notations:
         
-        1. Integer indices are currently not supported. This is because indexing by integer
-            returns a 1-d array instead of a 2-d array, which can possibly cause confusion.
-        2. Indexing by two lists or arrays with different lengths are supported. They are
-            treated to index rows and columns, such as ``exprs[[0,1,2], [3,4]]`` is perfectly
-            valid to index the first 3 cells with the fourth and fifth channel.
+        1. Integer indices are currently not supported. This is because indexing by
+           integer returns a 1-d array instead of a 2-d array, which can possibly cause confusion.
+        2. Indexing by two lists or arrays with different lengths are supported.
+           They are treated to index rows and columns, such as ``exprs[[0,1,2], [3,4]]`` is
+           perfectly valid to index the first 3 cells with the fourth and fifth channel.
         
         .. tip::