Documenting support for multiple stylesheets in javadoc

Javadoc now supports multiple stylesheets. A new command-line argument --add-stylesheet is introduced and the existing option -stylesheetfile will have a new alias --main-stylesheet.

Specification:

--add-stylesheet <path>

The option can be used one or more times to specify additional stylesheets to be included in the documentation.

Javadoc command-line example:

javadoc --add-stylesheet new_stylesheet_1.css --add-stylesheet new_stylesheet_2.css pkg_foo


-stylesheetfile <path> or --main-stylesheet <path>

In addition, the existing -stylesheetfile option will have an alias --main-stylesheet, with --main-stylesheet being the preferred form.

Javadoc command-line example:

javadoc --main-stylesheet main_stylesheet.css pkg_foo

The issue JDK-8185371 mentions the changes in detail. We need to document this new feature.