Update avoiding "instructions to spec authors" language

Co-authored-by: Chris Markiewicz <[email protected]>

src/common-principles.md

@@ -422,10 +422,11 @@ possible. Since the NIfTI standard offers limited support for the various image
 acquisition parameters available in DICOM files, we RECOMMEND that users provide
 additional meta information extracted from DICOM files in a sidecar JSON file
 (with the same filename as the `.nii[.gz]` file, but with a `.json` extension).
-Many of such metadata entries are explicitly formalized in BIDS and can be found listed in the [Glossary](./glossary.md).
-Whenever formalizing a new metadata entry within BIDS, the name SHOULD correspond to the original DICOM Tag whenever the value is taken *as is* without any further harmonization.
-The description of the term in the schema then SHOULD describe such correspondence as "Corresponds to DICOM Tag ID1, ID2 `DICOM Tag Name`.".
-If value is harmonized, the description SHOULD describe such correspondence as "Based on DICOM Tag ID1, ID2 `DICOM Tag Name`.".
+Currently defined metadata fields are listed in the [Glossary](./glossary.md).
+Where possible, DICOM Tags are adopted directly as BIDS metadata terms and
+indicated with "**Corresponds to** DICOM Tag ID1, ID2 `DICOM Tag Name`.".
+When harmonization has been deemed necessary, this is indicated in the
+BIDS term description with "**Based on** DICOM Tag ID1, ID2 `DICOM Tag Name`.".
 Extraction of BIDS compatible metadata can be performed using [dcm2niix](https://github.com/rordenlab/dcm2niix)
 and [dicm2nii](https://www.mathworks.com/matlabcentral/fileexchange/42997-xiangruili-dicm2nii)
 DICOM to NIfTI converters. The [BIDS-validator](https://github.com/bids-standard/bids-validator)