Update googletest docs to indicate that RE2 is now used for regexp...

Update googletest docs to indicate that RE2 is now used for regexp implementation when building with Bazel and using Abseil.

PiperOrigin-RevId: 454012698
Change-Id: I3a9df78368ffe3cb7cf3d315001125b97b1aebbe

docs/advanced.md

@@ -482,9 +482,11 @@ TEST_F(FooDeathTest, DoesThat) {
 
 ### Regular Expression Syntax
 
-On POSIX systems (e.g. Linux, Cygwin, and Mac), googletest uses the
+When built with Bazel and using Abseil, googletest uses the
+[RE2](https://github.com/google/re2/wiki/Syntax) syntax. Otherwise, for POSIX
+systems (Linux, Cygwin, Mac), googletest uses the
 [POSIX extended regular expression](http://www.opengroup.org/onlinepubs/009695399/basedefs/xbd_chap09.html#tag_09_04)
-syntax. To learn about this syntax, you may want to read this
+syntax. To learn about POSIX syntax, you may want to read this
 [Wikipedia entry](http://en.wikipedia.org/wiki/Regular_expression#POSIX_Extended_Regular_Expressions).
 
 On Windows, googletest uses its own simple regular expression implementation. It