I know that very few employers would take the risk to be so frank but I personally wish they were. It would be helpful to the candidates who actually care about improving their knowledge and skills while potentially offending those who are trying to fake it.
I've told candidates variants on that theme before.
"This position requires knowledge of programming, particularly in (C, Python, insert details here)."
Dancing around that point is not a "politeness".
That was, incidentally, sekasi's point #3, several parent references up:
"3. Always respond to requests for more information"
No. :)
"3. Within reason, always respond to requests for more information."
Many communication "rules" require the ability to moderate one's interpretation of the rule. I don't think this is any different. It's a good thing to give candidates feedback and if they're confused, the feedback did no good so you should be willing to help them understand. But if they're just unwilling to see the world or themselves for what they are, it's out of your hands and you should leave it well enough alone.
Following up on some references before calling people in for an interview isn't a bad idea.
Man, I have seen some blatantly incompetent ones. You'd give them a white board and ask them to show a linked list with boxes and arrows. They just couldn't do it.
One case I remember couldn't demonstrate pseudo code for a circular buffer operation based on an array with a head and tail index.
And (hopefully) how well they did in the phone screen...?
Hash tables might be a better filter, because IMHO you're a lot more likely to find those in real use these days than linked lists.
(The job area was embedded development, involving C and C++, so asking for C code related to linked lists would have been fair game!)
(These werr people who couldn't program FizzBuzz)
Plus anyone can just make shit up on their resume.