[#26] object: Rename UserHeader to Attribute

The name of object.UserHeader message implies its use by the user. However,
the attributes of an object can also be set internally by the system. To
generalize the intended purpose of the message, this commit renames it to an
Attribute (like container.Attribute).

Signed-off-by: Leonard Lyubich <leonard@nspcc.ru>

proto-docs/object.md

@@ -29,6 +29,7 @@
 - [object/types.proto](#object/types.proto)
 
   - Messages
+    - [Attribute](#object.Attribute)
     - [CreationPoint](#object.CreationPoint)
     - [Header](#object.Header)
     - [IntegrityHeader](#object.IntegrityHeader)
@@ -38,7 +39,6 @@
     - [SystemHeader](#object.SystemHeader)
     - [Tombstone](#object.Tombstone)
     - [Transform](#object.Transform)
-    - [UserHeader](#object.UserHeader)
     
 
 - [Scalar Value Types](#scalar-value-types)
@@ -361,6 +361,18 @@ in distributed system.
  <!-- end services -->
 
 
+<a name="object.Attribute"></a>
+
+### Message Attribute
+Attribute groups the parameters of the object attributes.
+
+
+| Field | Type | Label | Description |
+| ----- | ---- | ----- | ----------- |
+| Key | [string](#string) |  | Key carries the string key to the object attribute. |
+| Value | [string](#string) |  | Value carries the string value of the object attribute. |
+
+
 <a name="object.CreationPoint"></a>
 
 ### Message CreationPoint
@@ -383,7 +395,7 @@ in distributed system.
 | ----- | ---- | ----- | ----------- |
 | Link | [Link](#object.Link) |  | Link to other objects |
 | Redirect | [refs.Address](#refs.Address) |  | Redirect not used yet |
-| UserHeader | [UserHeader](#object.UserHeader) |  | UserHeader is a set of KV headers defined by user |
+| Attribute | [Attribute](#object.Attribute) |  | Attribute is a set of K-V object attributes |
 | Transform | [Transform](#object.Transform) |  | Transform defines transform operation (e.g. payload split) |
 | Tombstone | [Tombstone](#object.Tombstone) |  | Tombstone header that set up in deleted objects |
 | Token | [service.Token](#service.Token) |  | Token header contains token of the session within which the object was created |
@@ -475,18 +487,6 @@ in distributed system.
 | ----- | ---- | ----- | ----------- |
 | type | [Transform.Type](#object.Transform.Type) |  | Type of object transformation |
 
-
-<a name="object.UserHeader"></a>
-
-### Message UserHeader
-
-
-
-| Field | Type | Label | Description |
-| ----- | ---- | ----- | ----------- |
-| Key | [string](#string) |  | Key of the user's header |
-| Value | [string](#string) |  | Value of the user's header |
-
  <!-- end messages -->