DOC add docstring under random_varname, all_subclasses functions (#129)

Contributes to #105

testbook/utils.py

@@ -3,10 +3,32 @@
 
 
 def random_varname(length=10):
+    """
+    Creates a random variable name as string of a given length.
+    This is used in testbook to generate temporary variables within the notebook.
+
+    Parameters
+    ----------
+        length (int)
+
+    Returns:
+    --------
+        random variable name as string of given length
+    """
     return ''.join(random.choice(string.ascii_lowercase) for _ in range(length))
 
 
 def all_subclasses(klass):
+    """
+    This is a function that returns a generator.
+
+    Inspects subclasses associated with a given class in a recursive manner
+    and yields them, such that subclasses of subclasses will be yielded.
+
+    Parameters:
+    -----------
+        klass
+    """
     for subklass in klass.__subclasses__():
         yield subklass
         yield from all_subclasses(subklass)