The Mercurial interface there is quite confusing if you are not used to it, and since this is a large project, it can be hard to find what you are looking for.
Here is an example:
To find the JDK6 implementation java.util.List, select jdk6, jdk, select browse. Then browse to src/share/classes/java/util/List.java.
Grepcode.com is great for similar things - not only OpenJDK sources, with searching in classes/methods and links between classes directly in highlighted code:
zip the folder "classes/", make "classes/" as the root dir of .zip file, (e.g first cd jdk-2ab13901d6f1/src/share/, then zip -r openjdk_8u73_b2_src.zip classes/)
move the created zip file to proper location, it will stay there for a while, (e.g mv openjdk_8u73_b2_src.zip /media/Eric/software/java/jdk/openjdk/openjdk8u73-b02/source/)
in eclipse, specify source file for jars of installed jre, could specify the source attachment for each jar of installed jre respectively, the most common jar is probably rt.jar,
optionally, might need refresh project to make it totally take effect, not sure is that necessary,
test it: in eclipse, ctrl + shift + t, then input Cancellable, select the sun.nio.fs.Cancellable of corresponding installed jre,
if the source code is available, then it's good, because this source is not available in jdk_home/src.zip, it must be from the additional openjdk source,
switch source back: could switch back to use "jdk_home/src.zip", if don't want to use the external openjdk source,