[#1247] *: Clarify docs about returned errors from apistatus package

`apistatus` package provides types which implement build-in `error`
interface. Add `error of type` pattern when documenting these errors in
order to clarify how these errors should be handled (e.g. `errors.Is` is
not good).

Signed-off-by: Leonard Lyubich <leonard@nspcc.ru>

pkg/local_object_storage/writecache/delete.go

@@ -12,7 +12,7 @@ import (
 
 // Delete removes object from write-cache.
 //
-// Returns apistatus.ObjectNotFound is object is missing in write-cache.
+// Returns an error of type apistatus.ObjectNotFound if object is missing in write-cache.
 func (c *cache) Delete(addr *addressSDK.Address) error {
 	c.modeMtx.RLock()
 	defer c.modeMtx.RUnlock()