org-info: Try info index if info node is not found

org-info: Try info index if info node is not found

* lisp/org-info.el (org-info-follow-link): Attempt index lookup if node
  lookup fails.

* doc/org.texi (External links): Update info links.

Info index is almost always finer grain than info nodes.  For example
with this change, [[info:libc#close]] brings up not only
"(libc)Opening and Closing Files" info node, but also place the cursor
on the line that documents "close" function within the node.  This is
done by looking up "close"in the index upon failing to find a node
named "close".  Hence one can now link function, variable and other
names that are in the index rather than being limited to info node
names.  Typically there are far more index items than there are node
names.  For example libc manual has about 700 nodes, but over 4000
concept, type, function, and variables index items.  More examples of
new ways to create links are shown using org.info as example

- [[info:org#org-clock-idle-time]] uses "Variable Index"
- [[info:org#org-capture]] uses "Command and Function Index".
- [[info:org#timestamp]] uses "Main Index"
- [[info:org#C-c C-c]] uses "Key Index"

TINYCHANGE