Move description of defaults to separate section

Since the discussion also applies to the struct codec.

docs/metadata.md

@@ -101,6 +101,15 @@ using the Python API.
 .. todo:: Add examples of adding table-level metadata schemas.
 ```
 
+### Defaults
+
+```{eval-rst}
+.. todo:: Add examples of setting default values.
+```
+
+Default values for properties can be specified for only
+the shallowest level of the metadata object. 
+
 (sec_metadata_codecs)=
 
 ## Codecs
@@ -121,8 +130,7 @@ When `json` is specified as the `codec` in the schema the metadata is encoded in
 the human readable [JSON](https://www.json.org/json-en.html) format. As this format
 is human readable and encodes numbers as text it uses more bytes than the `struct`
 format. However it is simpler to configure as it doesn't require any format specifier
-for each type in the schema. Default values for properties can be specified for only
-the shallowest level of the metadata object. Tskit deviates from standard JSON in that
+for each type in the schema. Tskit deviates from standard JSON in that
 empty metadata is interpreted as an empty object. This is to allow setting of a schema
 to a table with out the need to modify all existing empty rows.