Fix gh-977 feedback: Module vshard

Revise info on bucket id field and shard index

(cherry picked from commit 878689a)

Author: lenkis

doc/reference/reference_rock/vshard/vshard_admin.rst

@@ -505,10 +505,11 @@ For example:
         box.schema.func.create('customer_add')
     end)
 
-Every space you plan to shard must have ``bucket_id`` unsigned field indexed
-by ``bucket_id`` TREE index. Spaces without ``bucket_id`` index don't
-participate in a sharded Tarantool cluster and can be used as regular
-spaces if needed.
+.. NOTE::
+
+    Every space you plan to shard must have a field with
+    :ref:`bucket id <vshard-vbuckets>` numbers, indexed by the
+    :ref:`shard index <cfg_basic-shard_index>`.
 
 .. _vshard-adding-data:
 

doc/reference/reference_rock/vshard/vshard_architecture.rst

@@ -76,10 +76,15 @@ multiple replica sets at a time.
 The total number of buckets is determined by the administrator who sets up the
 initial cluster configuration.
 
-Every Tarantool space you plan to shard must have a bucket id field indexed by the
-bucket id index. Spaces without bucket id indexes do not participate in sharding
-but can be used as regular spaces. By default, the name of the index coincides with
-the bucket id.
+Every space you plan to shard must have a numeric field containing bucket id-s.
+This field must comply with the following requirements:
+
+* The field's data type can be: unsigned, number or integer.
+* The field must be not nullable.
+* The field must be indexed by the :ref:`shard_index <cfg_basic-shard_index>`.
+  The default name for this index is ``bucket_id``.
+
+See the :ref:`configuration example <vshard-define-spaces>`.
 
 .. _vshard-structure:
 

doc/reference/reference_rock/vshard/vshard_ref.rst

@@ -46,10 +46,12 @@ Basic parameters
 
 .. confval:: shard_index
 
-    An index over the bucket id.
+    Name of a TREE index over the :ref:`bucket id <vshard-vbuckets>`.
+    Spaces without this index do not participate in a sharded Tarantool
+    cluster and can be used as regular spaces if needed.
 
     | Type: non-empty string or non-negative integer
-    | Default: coincides with the bucket id number
+    | Default: "bucket_id"
     | Dynamic: no
 
 .. _cfg_basic-bucket_count: