Explicitly warn about pitfalls with the auto keyword in C++11

Submitted by Christoph Hertzberg @chhtz

Assigned to Nobody

Link to original bugzilla bug (#825)
Version: 3.2

Description

Storing expression trees using C++11's auto keyword can lead to strange pitfalls, non-working or inefficient code.
Some examples are bug #434 (closed), bug #505 (closed), bug #680 (closed) and various mailing list questions.

Some non-working code will be fixed by bug #99 (closed), but still some inefficient code will remain -- while sometimes the use of auto can be valid.

I'd think the best we can do for now is to explain this issue prominently in our documentation and link to it from as many places as possible.

Blocking

#558 (closed) #505 (closed)