RFR: docs/build: JDK-8224166: Create a taglet to better handle @jls and @jvms tags

[Jonathan Gibbons]

Please review a small change to add support for new taglets to handle 
the @jls and @jvms tags found in our API docs.

Previously, these tags were just handled with simple javadoc 
command-line options to echo their contents.  Now, the section number is 
found in the contents, and a link to the standard location for the full 
appropriate online spec is generated.

Note that the links anticipate the publication of the updated versions 
of these specs in the standard location, and will be 404 until the 
versions are available, at GA time.  If you want to preview/play with 
the links, you can tweak the version number use in Docs.gmk, line 278. 
It defaults to the current $$(VERSION_SPECIFICATION), meaning that it 
will be updated automatically as we move forward to new versions.

The code for handling @jls and @jvms is sufficiently similar that they 
are implemented as nested classes within a single common supertype.   Of 
note is that the taglets do support the use of {@code} within the text.

...

Separately, you may have noticed some cleanup changesets to standardize 
the use of these tags. Although those changes are related to this work, 
it is _not_ a requirement that the standard form is used.  The section 
number is extracted in the taglet with a fairly tolerant regex.

-- Jon


JBS: https://bugs.openjdk.java.net/browse/JDK-8224166
Webrev: http://cr.openjdk.java.net/~jjg/8224166-jls-jvms/webrev.00/
API: http://cr.openjdk.java.net/~jjg/8224166-jls-jvms/docs/api/index.html