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

Author: Abseil Team

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