Verify Matcher vs. OpenJDK 8u121-b13, step 6.

Reinstate documentation for referencing named groups.

These two lines of documentation now exactly match upstream
OpenJDK 8u121-b13. The documentation had previously been omitted
because Android didn't use to support named groups, but such
support was added for Android 8.0 Oreo in libcore CL [1].

[1] commit 69860bf2 http://r.android.com/321531

Test: Treehugger
Bug: 80416774

Change-Id: Icdc2b4e979f9ec93ef0b3465f0ccf181695d8d41