The first paragraph says "Releases all unused cache entries from all caches." but doesn't list the specific caches. You can see the caches using DMV's. The view sys.dm_os_memory_cache_counters lists the individual caches and has counts of memory allocated and in use, entries allocated and entries in use. Querying the view before and after using DBCC FREESYSTEMCACHE ('ALL') shows it at work.
First use SELECT DISTINCT TYPE FROM SYS.DM_OS_MEMORY_CACHE_COUNTERS to see there are 27 cache types.
CACHESTORE_PHDR
CACHESTORE_XMLDBTYPE
CACHESTORE_EVENTS
USERSTORE_OBJPERM
USERSTORE_TOKENPERM
CACHESTORE_FULLTEXTSTOPLIST
CACHESTORE_XPROC
CACHESTORE_BROKERRSB
CACHESTORE_XMLDBATTRIBUTE
CACHESTORE_STACKFRAMES
CACHESTORE_NOTIF
CACHESTORE_XMLDBELEMENT
CACHESTORE_OBJCP
CACHESTORE_CONVPRI
CACHESTORE_BROKERREADONLY
CACHESTORE_SQLCP
CACHESTORE_SYSTEMROWSET
USERSTORE_SCHEMAMGR
CACHESTORE_TEMPTABLES
CACHESTORE_BROKERTBLACS
CACHESTORE_BROKERTO
CACHESTORE_BROKERKEK
USERSTORE_SXC
CACHESTORE_BROKERUSERCERTLOOKUP
CACHESTORE_BROKERDSH
USERSTORE_DBMETADATA
CACHESTORE_VIEWDEFINITIONS
Most of these have a single cache associated with them (EG CACHSTORE_PHDR has one cache called Bound Trees). Some have multiple caches, eg USERSTORE_TOKENPERM has hundreds of caches but most of the entries are in a few of them like the TokenAndPermUserStore.
Compare this to FREEPROCCACHE: "Removes all elements from the plan cache...". This only affects the CACHESTORE_OBJCP and
CACHESTORE_SQLCP caches.