[PATCH] Add limitations to loop convert user doc

[Sean Silva]

  > @silvas, regarding the "Iterator" subheading: My intention for that section was to talk about how the range-based for loop calls end() only once during the initialization of the loop, so if the original header called end() after each iteration, semantics may change.

  Please give it a title that indicates that then!

  I would also suggest looking at the other sections too and see how/if their titles could be improved to reflect their content better. E.g. "comments" would probably be better as "comments inside ``for`` header are not preserved". A good title can save the user a lot of reading/rereading by setting their expectations for the section. Additionally, it makes the table of contents much more effective: Compare

  * Limitations
  ** Comments
  ** Iterators
  ** ...

  vs

  * Limitations
  ** Comments in the for header are not preserved
  ** range-for evaluates end()/size()/operator*() only once
  ** ...

http://llvm-reviews.chandlerc.com/D552