Clarify NCD filename without directory (#322)

sigmf-schema.json

@@ -34,7 +34,7 @@
           "type": "string"
         },
         "core:dataset": {
-          "description": "The full filename of the Dataset file this Metadata file describes, used ONLY with Non-Conforming Datasets. If provided, this string MUST be the complete filename of the Dataset file, including the extension. The Dataset file must be in the local directory, and this string MUST NOT include any aspects of filepath other than the filename.  If a Recording does not have this field, it MUST have a compliant SigMF Dataset (NOT a Non-Conforming Dataset) which MUST use the same base filename as the Metadata file and use the `.sigmf-data` extension. If a SigMF Recording or Archive is renamed this field MUST also be updated, because of this it is RECOMMENDED that Compliant SigMF Recordings avoid use of this field.  This field SHOULD NOT be used in conjunction the `core:metadata_only` field. If both fields exist and the file specified by `core:dataset` exists, then `core:metadata_only` SHOULD be ignored by the application.",
+          "description": "The full filename of the Dataset file this Metadata file describes, used ONLY with Non-Conforming Datasets. If provided, this string MUST be the complete filename of the Dataset file, including the extension. The Dataset file must be in the same directory as the .sigmf-meta file; note that this string only includes the filename, not directory.  If a Recording does not have this field, it MUST have a compliant SigMF Dataset (NOT a Non-Conforming Dataset) which MUST use the same base filename as the Metadata file and use the `.sigmf-data` extension. If a SigMF Recording or Archive is renamed this field MUST also be updated, because of this it is RECOMMENDED that Compliant SigMF Recordings avoid use of this field.  This field SHOULD NOT be used in conjunction the `core:metadata_only` field. If both fields exist and the file specified by `core:dataset` exists, then `core:metadata_only` SHOULD be ignored by the application.",
           "type": "string",
           "pattern": "^[^\\/\\\\:*?\"<>|]+(\\.[^\\/\\\\:*?\"<>|]+)*\\$"
         },