doc: raise doxygen lookup cache

At the end of a run, doxygen emits a notice regarding the cache lookup
size being too small.  This cache is used to find symbols given their
name which would speed up processing a bit.

The notice before this patch:

lookup cache used 65536/65536 hits=2258467 misses=112595
Note: based on cache misses the ideal setting for LOOKUP_CACHE_SIZE is 1
at the cost of higher memory usage.

And after:

lookup cache used 104632/131072 hits=2131285 misses=105434

That is 7% less miss, not that much but still.  Note how the cache is
well filled (80%).

Change-Id: I0b97e8c5581a63c44a382891f8f1fc8d60975e80