[NFC][ntuple] add clarifying comment

tree/ntuple/v7/doc/specifications.md

@@ -233,7 +233,7 @@ In this case, the locator should be interpreted like a frame, i.e. size indicate
 _Offset_:
 For on-disk / in-file locators, the 64bit byte offset of the referenced byte range counted from the start of the file.
 
-For non-disk locators, i.e. `T` == 1, the locator format is as follows
+For non-standard locators, i.e. `T` == 1, the locator format is as follows
 
 ```
  0                   1                   2                   3
@@ -266,10 +266,10 @@ followed by a locator.
 
 ### Well-known Payload Formats
 
-This section describes the well-known payload formats used in non-disk locators.
+This section describes the well-known payload formats used in non-standard locators.
 Note that locators having a different value for _Type_ may share a given payload format (see the table above).
 
-- _Large_: Like the standard on-disk locator but with a 64bit offset
+- _Large_: Like the standard on-disk locator but with a 64bit size
 ```
  0                   1                   2                   3
  0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1

tree/ntuple/v7/inc/ROOT/RNTupleUtil.hxx

@@ -197,6 +197,8 @@ struct RNTupleLocator {
    /// Values for the _Type_ field in non-disk locators.  Serializable types must have the MSb == 0; see
    /// `doc/specifications.md` for details
    enum ELocatorType : std::uint8_t {
+      // The kTypeFile locator may translate to an on-disk standard locator (type 0x00) or a large locator (type 0x01),
+      // if the size of the referenced data block is >2GB
       kTypeFile = 0x00,
       kTypeDAOS = 0x02,