docs: minor comment update for Entity message (#1105)

* docs: minor comment update for Entity message

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handwritten/nodejs-datastore/protos/google/datastore/v1/entity.proto

@@ -197,9 +197,7 @@ message Value {
 
 // A Datastore data object.
 //
-// An entity is limited to 1 megabyte when stored. That _roughly_
-// corresponds to a limit of 1 megabyte for the serialized form of this
-// message.
+// Must not exceed 1 MiB - 4 bytes.
 message Entity {
   // The entity's key.
   //