Add comment about pruning

man/memoryCache.Rd

@@ -30,6 +30,14 @@ environment. Objects can be stored and retrieved using the \code{get()} and
 according to the parameters \code{max_size}, \code{max_age}, \code{max_n},
 and \code{evict}.
 }
+\details{
+In a \code{MemoryCache}, R objects are stored directly in the cache; they are
+not \emph{not} serialized before being stored in the cache. This contrasts
+with other cache types, like \code{\link{DiskCache}}, where objects are
+serialized, and the serialized object is cached. This can result in some
+differences of behavior. For example, as long as an object is stored in a
+MemoryCache, it will not be garbage collected.
+}
 \section{Cache pruning}{