doc: tuning: note reduced fragmentation w/ jemalloc

I may be mistaken, but I suspect the reason jemalloc handles
long-lived processes better than glibc is due to granularity
reduction being scaled to larger size classes.  This can waste
20% of an individual allocation, but increases the likelyhood
of reuse (without splitting/consolidating into other sizes).

In other words, glibc seems to try too hard to make the best fit
for initial allocations.  This ends up being suboptimal over
time as those allocations are freed and similar (but not
identical) allocations come in.  jemalloc sacrifices the best
initial fit for better fits over a long process lifetime.

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