Mark regexp feature as preview

Part of #130.

lang/spec.html

@@ -42,6 +42,10 @@ <h1>Ballerina Language Specification, 2022R4 preview</h1>
 make changes that introduce significant incompatibilities.
 </p>
 <p>
+This release contains a preview of the regular expression feature. Minor
+changes may be made before it becomes stable.
+</p>
+<p>
 Comments on this document are welcome and should be made by creating an issue in
 <code><a href="https://github.com/ballerina-platform/ballerina-spec"
 >https://github.com/ballerina-platform/ballerina-spec</a></code>, which is the
@@ -10841,7 +10845,7 @@ <h3>Ballerina Flavored Markdown</h3>
 <section>
 <h2 id="data_tags">10. Data tags</h2>
 <section>
-<h3>Regular expressions</h3>
+<h3>Regular expressions (preview)</h3>
 
 <p>
 The <code>re</code> data tag is used for regular expressions. The lang library
@@ -11287,7 +11291,7 @@ <h4 id="lang.object"><code>lang.object</code> module</h4>
 </section>
 
 <section>
-<h4 id="lang.regexp"><code>lang.regexp</code> module</h4>
+<h4 id="lang.regexp"><code>lang.regexp</code> module (preview)</h4>
 
 <p>The <code>lang.regexp</code> module corresponds to the basic type for the
 tagged data type with tag <code>re</code>.</p>
@@ -11390,7 +11394,8 @@ <h2 id="changes">B. Changes since previous releases</h2>
 <section>
 <h3>Summary of changes from 2022R3 to 2022R4</h3>
 <ol>
-<li>Regular expressions have been added as a new data type.</li>
+<li>Regular expressions have been added as a new data type. This
+feature has preview status in this release.</li>
 <li>A <code>byte-array-literal</code> can construct a read-only value.</li>
 <li>An optional semi-colon is now allowed after top-level definitions and
 method definitions that are currentlty terminated by <code>}</code>.</li>