Document thread safety of DiskStore and FlatFileStore

Signed-off-by: Graham <gpe@openrs2.dev>

cache/src/main/java/dev/openrs2/cache/DiskStore.kt

@@ -16,6 +16,8 @@ import kotlin.math.min
 /**
  * A [Store] implementation compatible with the native `main_file_cache.dat2`
  * and `main_file_cache.idx*` format used by the client.
+ *
+ * This class is not thread safe.
  */
 public class DiskStore private constructor(
     private val root: Path,

cache/src/main/java/dev/openrs2/cache/FlatFileStore.kt

@@ -14,6 +14,10 @@ import java.nio.file.Path
  * and groups as file system files. This format is much friendlier to
  * content-addressable version control systems, such as Git, than the native
  * format used by the client.
+ *
+ * Multiple read threads may use this class simultaneously. However, only a
+ * single thread may write at a time. Reads and writes must not happen
+ * simultaneously.
  */
 public class FlatFileStore private constructor(
     private val root: Path,