Document standard fields in signed portion for all metadata files

Be explicit that standard fields in the signed portion match the
definitions for the same fields in root.json

Signed-off-by: Joshua Lock <[email protected]>

Author: joshuagl

tuf-spec.md

@@ -724,6 +724,8 @@ repo](https://github.com/theupdateframework/specification/issues).
          "meta" : METAFILES
        }
 
+   SPEC_VERSION, VERSION and EXPIRES are the same as is described for the root.json file.
+
    METAFILES is an object whose format is the following:
 
        { METAPATH : {
@@ -798,6 +800,8 @@ repo](https://github.com/theupdateframework/specification/issues).
          ("delegations" : DELEGATIONS)
        }
 
+   SPEC_VERSION, VERSION and EXPIRES are the same as is described for the root.json file.
+
    TARGETS is an object whose format is the following:
 
        { TARGETPATH : {
@@ -981,6 +985,8 @@ repo](https://github.com/theupdateframework/specification/issues).
          "meta" : METAFILES
        }
 
+   SPEC_VERSION, VERSION and EXPIRES are the same as is described for the root.json file.
+
    METAFILES is the same as described for the snapshot.json file.  In the case
    of the timestamp.json file, this MUST only include a description of the
    snapshot.json file.