[11u] RFR: 8221894: Add comments for docker tests in the test doc

[Severin Gehwolf]

Hi,

Please review this doc-only OpenJDK 11u backport so as to get a more
clean backport of JDK-8222299. See discussion in [1]. The JDK 13 patch
does not apply cleanly since testing.html looks different. However,
that's a generated file (via "make update-build-docs") and
doc/testing.md applies cleanly. So the proposed webrev is applying
doc/testing.md changes as-is from JDK 13 and then use the generated
doc/testing.html changes after running "make update-build-docs".

Bug: https://bugs.openjdk.java.net/browse/JDK-8221894
webrev: http://cr.openjdk.java.net/~sgehwolf/webrevs/JDK-8221894/jdk11/01/webrev/
Original changeset: http://hg.openjdk.java.net/jdk/jdk/rev/e437ad5643d6

Note that added docs apply to OpenJDK 11u as well after tag jdk-11.0.5+3[2].

Thoughts?

Thanks,
Severin

[1] http://mail.openjdk.java.net/pipermail/jdk-updates-dev/2019-August/001682.html
[2] Among others:
    http://hg.openjdk.java.net/jdk-updates/jdk11u/rev/485b03da3c6d