DOC: Increase Doxygen LOOKUP_CACHE_SIZE.

This value is increased to the value suggested at the end of a doxygen build.

This increases doxygen build performance at the cost of memory usage.  The
cache size will still be below 300 MB.

Change-Id: I4d902bd751d000c9f861e21cf778dda66faeabc8
Merge topic 'doxygen'

ebb8beb0 DOC: Increase Doxygen LOOKUP_CACHE_SIZE.