RFR: JDK-8189056 javadoc target for stable specdiff comparisons

[Magnus Ihse Bursie]

 From the bug report: "For JSR 383 (Java SE 18.3), we need weekly diffs 
of the Java SE 18.3 API specification. The current "doc-javase" makefile 
target always uses the most recent version of the javadoc tool (i.e. the 
tool that was just built). Unfortunately, this introduces unwanted diffs 
that are due to changes in the javadoc tool's formatting of the 
specification. A means to use a fixed version of the javadoc tool so 
that the only detected changes are those to the specification itself is 
needed."

I have introduced a new class of documentation, "docs-reference", that 
will create up-to-date javadoc using the Boot JDK javadoc, and a fixed 
set of options.

Note that this bug is blocked on JDK-8189094 (Change required boot jdk 
to JDK 9). I will await that patch to enter jdk10/master before pushing 
this, but my preliminary testing indicates that there will be no issues. 
(However, with JDK 8 as Boot JDK, this will not work, since the command 
line options is not accepted by JDK 8 javadoc.) If it turns out that 
more fixing is needed, I'll reiterate this review before pushing.

Bug: https://bugs.openjdk.java.net/browse/JDK-8189056
WebRev: 
http://cr.openjdk.java.net/~ihse/JDK-8189056-reference-javadoc/webrev.01

/Magnus