Change NBTAttrs a bit so sphinx documents them quietly

Both of these changes make them more like the builtin property()
NBTAttr access on classes returns self, so sphinx can document the class
NBTAttr constructors take a docstring argument and replace the instance's doc so sphinx doesn't repeat NBTAttr's doc everywhere

src/mceditlib/nbtattr.py

@@ -1,5 +1,5 @@
 """
-    ${NAME}
+    nbtattr
 """
 from __future__ import absolute_import, division, print_function, unicode_literals
 import collections
@@ -27,7 +27,7 @@ class NBTAttr(object):
     The function SetNBTDefaults can be used to initialize all subtags of an instance's rootTag to their defaults,
     creating them if needed.
 
-    Compare:
+    Without NBTAttr::
 
         # getting
         if "Dimension" not in rootTag:
@@ -39,7 +39,7 @@ class NBTAttr(object):
             rootTag["Dimension"] = nbt.TAG_Int()
         rootTag["Dimension"].value = 1
 
-    With NBTAttrs:
+    With NBTAttr::
 
         class PlayerRef(object):
             Dimension = NBTAttr("Dimension", nbt.TAG_Int, 0)
@@ -59,12 +59,15 @@ class NBTAttr(object):
     def __repr__(self):
         return "NBTAttr('%s', %s, %r)" % (self.name, self.tagType, self.default)
 
-    def __init__(self, name, tagType, default=None):
+    def __init__(self, name, tagType, default=None, doc=""):
         self.name = name
         self.tagType = tagType
         self.default = default
+        self.__doc__ = doc
 
     def __get__(self, instance, owner):
+        if instance is None:
+            return self  # attribute access on class returns the NBTAttr, like property()
         tag = instance.rootTag
         if self.name not in tag:
             if self.default is None:
@@ -82,10 +85,16 @@ class NBTAttr(object):
 
 
 class NBTUUIDAttr(object):
+    """
+    This attribute gets the entity's UUID from its `UUIDLeast` and `UUIDMost` tags and
+    returns it as a `uuid.UUID`
+    """
     def __repr__(self):
         return "NBTUUIDAttr()"
 
     def __get__(self, instance, owner):
+        if instance is None:
+            return self
         tag = instance.rootTag
         if 'UUIDLeast' not in tag or 'UUIDMost' not in tag:
             return None
@@ -200,15 +209,18 @@ class NBTCompoundListAttr(object):
     to the constructor.
     """
 
-    def __init__(self, name, compoundRefClass):
+    def __init__(self, name, compoundRefClass, doc=""):
         self.name = name
         self.compoundRefClass = compoundRefClass
         self.listProxyClass = NBTListProxy
+        self.__doc__ = doc
 
     def __repr__(self):
         return "NBTListAttr(%s, %s)" % (self.name, self.compoundRefClass)
 
     def __get__(self, instance, owner):
+        if instance is None:
+            return self
         tag = instance.rootTag
         if self.name not in tag:
             tag[self.name] = nbt.TAG_List()
@@ -220,12 +232,15 @@ class NBTListAttr(object):
     def __repr__(self):
         return "NBTListAttr(%s, %s, %s)" % (self.name, self.listType, self.default)
 
-    def __init__(self, name, listType, default=()):
+    def __init__(self, name, listType, default=(), doc=""):
         self.name = name
         self.listType = listType
         self.default = default
+        self.__doc__ = doc
 
     def __get__(self, instance, owner):
+        if instance is None:
+            return self
         tag = instance.rootTag
         if self.name not in tag:
             tag[self.name] = nbt.TAG_List()
@@ -249,11 +264,13 @@ class NBTCompoundAttr(NBTAttr):
     def __repr__(self):
         return "NBTCompoundAttr(%s, %s)" % (self.name, self.compoundRefClass)
 
-    def __init__(self, name, compoundRefClass):
-        super(NBTCompoundAttr, self).__init__(name, nbt.TAG_Compound)
+    def __init__(self, name, compoundRefClass, doc=""):
+        super(NBTCompoundAttr, self).__init__(name, nbt.TAG_Compound, doc=doc)
         self.compoundRefClass = compoundRefClass
 
     def __get__(self, instance, owner):
+        if instance is None:
+            return self
         tag = instance.rootTag
         if self.name not in tag:
             tag[self.name] = self.tagType(value=self.default)
@@ -273,6 +290,9 @@ class NBTCompoundAttr(NBTAttr):
 
 class NBTVectorAttr(NBTListAttr):
     def __get__(self, instance, owner):
+        if instance is None:
+            return self
+
         val = super(NBTVectorAttr, self).__get__(instance, owner)
         return Vector(*val)
 
@@ -283,12 +303,15 @@ class KeyedVectorAttr(object):
     instead of as a list of tags. For example, the world spawn position (SpawnX, SpawnY,
     SpawnZ) and a TileEntity's position (x, y, z).
     """
-    def __init__(self, xKey, yKey, zKey, tagType, default=None):
+    def __init__(self, xKey, yKey, zKey, tagType, default=None, doc=""):
         self.tagType = tagType
         self.default = default
         self.keys = xKey, yKey, zKey
+        self.__doc__ = doc
 
     def __get__(self, instance, owner):
+        if instance is None:
+            return self
         tag = instance.rootTag
         for key in self.keys:
             if key not in tag: