add intro sentences

Author: rustagir

source/whats-new.txt

@@ -60,6 +60,9 @@ What's New in 1.20
    Support for {+mdb-server+} v3.6 is removed in this release of the
    library.
 
+The {+library-short+} v1.20 release includes the following features,
+improvements, and fixes:
+
 - Adds support for {+mdb-server+} v8.0.
 
 - Adds support for Queryable Encryption (QE) range queries. To use this
@@ -93,6 +96,9 @@ and was added to keep version parity with the {+extension-short+}.
 What's New in 1.18
 ------------------
 
+The {+library-short+} v1.18 release includes the following features,
+improvements, and fixes:
+
 - Adds a new GridFS API to make it more convenient to work with files using PHP's 
   existing filesystem functions. The :phpmethod:`MongoDB\GridFS\Bucket::registerGlobalStreamWrapperAlias()` 
   method may be used to register a global alias for a GridFS bucket. After 
@@ -114,6 +120,9 @@ To learn more about this release, see the `v1.18 Release Notes
 What's New in 1.17
 ------------------
 
+The {+library-short+} v1.17 release includes the following features,
+improvements, and fixes:
+
 - Introduces a new "codec" API for converting BSON to and from PHP objects. 
   More information on this feature may be found in the 
   :ref:`Codecs tutorial <php-codecs>`.
@@ -136,4 +145,4 @@ What's New in 1.17
   7.2 and 7.3 has been removed and the library now requires PHP 7.4 or newer.
 
 To learn more about this release, see the `v1.17 Release Notes 
-<https://github.com/mongodb/mongo-php-library/releases/tag/1.17.0>`__ on GitHub.
+<https://github.com/mongodb/mongo-php-library/releases/tag/1.17.0>`__ on GitHub.