To specify index properties in plugin.xml:
[EclipseHelpOptions]
; IdxFilename = name for index file, normally index.xml
IdxFilename=index.xml
; UseIndex = No (default) or Yes (for newer releases of Eclipse)
UseIndex = Yes
See §12.4.5 Configuring index properties for Eclipse Help.
12.3.4.4 Specifying Eclipse Help TOC properties in plugin.xml
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12.3.4.6 Including a runtime element in plugin.xml