add clear explanations for spect update [5D]

[remiadon]

• [x] update new_features.md
• [x] create issue for type indexing
• [x] create issue for "Activity definitions"
• [x] create issue for "Activities attributes"
• [x] create issue for Multiple entities as input
• [x] validate

[remiadon]

update new_features.md in 76bd4694b70bec7308bd84104552801e7094e65d

[remiadon]

Here are the related issues

• https://github.com/bids-standard/BEP028_BIDSprov/issues/43
• https://github.com/bids-standard/BEP028_BIDSprov/issues/44
• https://github.com/bids-standard/BEP028_BIDSprov/issues/45
• https://github.com/bids-standard/BEP028_BIDSprov/issues/46

[remiadon]

@cmaumet waiting for you to validate

44 and #45 are left as ongoing, as they were re-discussed yesterday

43 and #46 seem OK to me

[cmaumet]

This is excellent! Thanks @remiadon. Let's close this issue and continue the discussions in the relevant issues.