Bug 567974 - Add documentation for "showSearchDescription"

Add documentation for the new help system customization option:
org.eclipse.help.base/showSearchDescription

Also, minor rewording for "showSearchDescription" and
"showSearchCategories" to make it clearer.

Change-Id: Ia521d32d4ab66395cd7e8f370dc7695b8eb9cd6d
Signed-off-by: Holger Voormann <eclipse@voormann.de>