When creating a split manual with one-node-per-page, the main index.html
ends up getting clobbered by the page for the index() function because
it uses "@node index" which, for html, also creates an index.html page.
To remedy this, add "Function " to every function node so now we output
"Function-index.html" and avoid clobbering. It also namespaces every
other function and helps make sure we don't clobber anything else.
Otherwise, there isn't really much rendering difference as @node text
is mostly internal. Node title text comes from @section instead.
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