Update concepts.dd

The old version of isprime would return false for 2 and return true for any n with no factor less than sqrt(n).

These alterations fix that.
Update concepts.dd

in function isPrime: reversed the change to the condition in the loop. Changed name to isPrime in keeping with style guide.
Update concepts.dd

in isPrime: fixed braces style and corrected inequality.
Added missing dup property

For some reason dup wasn't documented for AAs.
Clarify wording of D variadic functions

It said that _argptr "enables the creation of typesafe variadic
functions". This is misleading, since the  next section is titled
"Typesafe variadic functions" but means something different!
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