remove obsolete reference to SVN trunk

googletest/docs/Documentation.md

-This page lists all documentation wiki pages for Google Test **(the SVN trunk version)**
+This page lists all documentation markdown files for Google Test **(the
+current git version)**
 -- **if you use a released version of Google Test, please read the
-documentation for that specific version instead.**
+documentation for that specific version instead (e.g. by checking out
+the respective git branch/tag).**
 
   * [Primer](Primer.md) -- start here if you are new to Google Test.
   * [Samples](Samples.md) -- learn from examples.