DOCSP-36707 improves Unique Indexes page (#7292)

* DOCSP-36707 improves Unique Indexes page

* external review

Author: jmd-mongo

source/core/index-unique.txt

@@ -24,18 +24,11 @@ Unique Indexes
    :class: singlecol
 
 A unique index ensures that the indexed fields do not store duplicate
-values; i.e. enforces uniqueness for the indexed fields. By default,
-MongoDB creates a unique index on the :ref:`_id <document-id-field>`
-field during the creation of a collection.
-
-.. note:: New Internal Format
-
-
-   Starting in MongoDB 4.2, for :ref:`featureCompatibilityVersion
-   <view-fcv>` (fCV) of 4.2 (or greater), MongoDB uses a new internal
-   format for unique indexes that is incompatible with earlier MongoDB
-   versions. The new format applies to both existing unique indexes as
-   well as newly created/rebuilt unique indexes.
+values. A unique index on a single field ensures that a value appears 
+at most once for a given field. A unique compound index ensures that 
+any given *combination* of the index key values only appears at most 
+once. By default, MongoDB creates a unique index on the :ref:`_id 
+<document-id-field>` field during the creation of a collection.
 
 .. |page-topic| replace:: :atlas:`create and manage unique indexes in the UI </atlas-ui/indexes>`
 
@@ -77,9 +70,9 @@ Unique Compound Index
 ~~~~~~~~~~~~~~~~~~~~~
 
 You can also enforce a unique constraint on :ref:`compound indexes
-<index-type-compound>`. If you use the unique constraint on a
-:ref:`compound index <index-type-compound>`, then MongoDB will enforce
-uniqueness on the *combination* of the index key values.
+<index-type-compound>`. A unique :ref:`compound index 
+<index-type-compound>` enforces uniqueness on the *combination* of the 
+index key values.
 
 For example, to create a unique index on ``groupNumber``, ``lastname``,
 and ``firstname`` fields of the ``members`` collection, use the
@@ -139,18 +132,17 @@ index <index-type-hashed>`.
 Building Unique Index on Replica Sets and Sharded Clusters
 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
 
-For replica sets and sharded clusters, using a :doc:`rolling procedure
-</tutorial/build-indexes-on-replica-sets>` to create a unique index
+For replica sets and sharded clusters, using a :ref:`rolling procedure
+<index-build-on-replica-sets>` to create a unique index
 requires that you stop all writes to the collection during the
 procedure. If you cannot stop all writes to the collection during the
-procedure, do not use the rolling procedure. Instead, build your unique index
-on the collection by:
-
-- issuing :method:`db.collection.createIndex()` on the primary for a
-  replica set, or
+procedure, do not use the rolling procedure. Instead, to build your 
+unique index on the collection you must either:
 
-- issuing :method:`db.collection.createIndex()` on the
-  :binary:`~bin.mongos` for a sharded cluster.
+- Run ``db.collection.createIndex()`` on the primary for a
+  replica set
+- Run ``db.collection.createIndex()`` on the :binary:`~bin.mongos` 
+  for a sharded cluster
 
 .. _unique-separate-documents: