Reword GSD_CHARACTER definition in docs.

doc/file-layer.rst

@@ -6,7 +6,7 @@ File layer
 
 .. highlight:: c
 
-**Version: 2.0**
+**Version: 2.x**
 
 General simulation data (GSD) **file layer** design and rationale. These use
 cases and design specifications define the low level GSD file format.
@@ -236,5 +236,8 @@ A data block stores raw data bytes on the disk. For a given index entry
 ``entry``, the data starts at location ``entry.location`` and is the next
 ``entry.N * entry.M * gsd_sizeof_type(entry.type)`` bytes.
 
-The type fields are self-explanatory except for UTF-8 strings, for which
-null-termination is allowed but not required.
+Added in version 2.1
+^^^^^^^^^^^^^^^^^^^^
+
+* The ``GSD_CHARACTER`` chunk type represents a UTF-8 string (null termination is allowed, but not
+  required).