Update Bucket name docstring to define valid values.

Author: daspecster

storage/google/cloud/storage/_helpers.py

@@ -30,7 +30,8 @@ class _PropertyMixin(object):
       - path
 
     :type name: str
-    :param name: The name of the object.
+    :param name: The name of the object. Bucket names must start and end with a
+                 number or letter.
     """
 
     def __init__(self, name=None):

storage/google/cloud/storage/bucket.py

@@ -81,7 +81,8 @@ class Bucket(_PropertyMixin):
                    for the bucket (which requires a project).
 
     :type name: str
-    :param name: The name of the bucket.
+    :param name: The name of the bucket. Bucket names must start and end with a
+                 number or letter.
     """
 
     _MAX_OBJECTS_FOR_ITERATION = 256