Guarded allocator: Document non-obvious initialization with MEM_new()

Initialization with `MEM_new()` depends a lot on the initialization rules
of C++, which are not obvious. Calling it with no arguments to be passed
to the constructor may cause the resulting object to be implicitly 0
initialized (or parts of it). This can have an impact on performance
sensitive code, so it's something to document.

Alternatively we could enforce default initialization (as opposed to the
value initalization that happens now), but this could cause
uninitialized memory in a way that isn't obvious either. This is a
possible source of bugs, so Jacques and I decided against it.

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