Document cache information sysconf variables

Write short descriptions for each of the cache information sysconf
variables.

	* manual/conf.texi (_SC_LEVEL1_ICACHE_SIZE,
	_SC_LEVEL1_ICACHE_ASSOC, _SC_LEVEL1_ICACHE_LINESIZE,
	_SC_LEVEL1_DCACHE_SIZE, _SC_LEVEL1_DCACHE_ASSOC,
	_SC_LEVEL1_DCACHE_LINESIZE, _SC_LEVEL2_CACHE_SIZE,
	_SC_LEVEL2_CACHE_ASSOC, _SC_LEVEL2_CACHE_LINESIZE,
	_SC_LEVEL3_CACHE_SIZE, _SC_LEVEL3_CACHE_ASSOC,
	_SC_LEVEL3_CACHE_LINESIZE, _SC_LEVEL4_CACHE_SIZE,
	_SC_LEVEL4_CACHE_ASSOC, _SC_LEVEL4_CACHE_LINESIZE): New
	variables.

Reviewed-by: Rical Jasan <ricaljasan@pacific.net>