Document see and see-also indexterm attributes

Original issue: https://github.com/asciidoctor/asciidoc-docs/issues/120

In brief, the shorthand delimiter >> defines a see term and the shorthand delimiter &> defines one or more see also terms.

Here's an example of an index term with a see term (i.e., redirect):

(((Flash >> HTML 5)))

This is shorthand for:

indexterm:[Flash,see=HTML 5]

Here's an example of an index term with see also terms:

(((HTML 5 &> CSS 3 &> SVG)))

This is shorthand for:

indexterm:[HTML 5,see-also="CSS 3,SVG"]

Both attributes work with the visible index term as well (double round brackets or indexterm2 macro).

There can only be one "see" term. There can be multiple "see also" terms.