[expr.new] Avoid "heap allocation"

The term "heap allocation" is not normatively defined, and the term "heap" has other meaning in the standard. Here "heap allocation" is typical implementation details, likely close to "free store". To clarify the intent, it would be better never used at all, even in an example.
Merge pull request #380 from FrankHB/patch-3

[expr.new] Avoid "heap allocation"