RFR: JDK-8241470 HtmlStyle: group and document members: description, flex, signature

[Jonathan Gibbons]

Please review a reasonably simple, mostly doc-only change to reorder and 
document some groups of members of HtmlStyle.

The change is primarily to document members in the conceptual groups 
regarding member signatures, flex layout, and descriptions/notes. Other 
potential groups will be identified and done separately.

The only code change is to rename `arguments` to `parameters`, in 
accordance with standard usage. This requires a corresponding change in 
the style sheet, and some updates to tests. The test updates were all 
done automagically.

-- Jon

JBS: https://bugs.openjdk.java.net/browse/JDK-8241470
Webrev: http://cr.openjdk.java.net/~jjg/8241470/webrev.00/index.html