DOCS-13254: 3.6.16 PCRE version is 8.42

Author: kay-kim

source/reference/operator/query/regex.txt

@@ -20,8 +20,13 @@ Definition
 .. query:: $regex
 
    Provides regular expression capabilities for pattern matching
-   *strings* in queries. MongoDB uses Perl compatible regular
-   expressions (i.e. "PCRE" ) version 8.41 with UTF-8 support.
+   *strings* in queries. In MongoDB 3.6-series, 
+   
+   - 3.6.16+ use Perl compatible regular expressions (i.e.
+     "PCRE" ) version 8.42 with UTF-8 support.
+
+   - 3.6.0-3.6.15 use Perl compatible regular expressions (i.e.
+     "PCRE" ) version 8.41 with UTF-8 support.
 
    To use :query:`$regex`, use one of the following syntaxes: