making "Entry for index `%s' outside of any node" a warning?

[Patrice Dumas]

Hello,

When there is an index entry outside of any node or section, there is an
error message:

  Entry for index `%s' outside of any node

I have just run into a real world manual (in auctex) which has such an 
index entries, because a file that has no node nor section can either be
processed within a manual, with an @include within a node (no error in 
that case), or standalone, to produce a text file. 

So I propose to transform that to a warning.

Agreed?

-- 
Pat