Document that built-in signals use send()

[Kevin Christopher Henry]

I think that Django consistently uses send() rather than send_robust() for its built-in signals, but it's not mentioned in ​the documentation.

The distinction matters to application code that uses these signals, since it determines whether or not a raised Exception will propagate.

[Filipa Andrade]

Added documentation available in ​https://github.com/filias/django/tree/ticket_26493

[Tim Graham <timograham@…>]

In 47fbbc33:

Fixed #26493 -- Documented how built-in signals are sent.

[Tim Graham <timograham@…>]

In 8bf5ced:

[1.9.x] Fixed #26493 -- Documented how built-in signals are sent.

Backport of 47fbbc33de805c803c39483344854caa2890c32c from master