Make wording more explicit for _mod and remove MRI specificity of _acq

[yarikoptic]

_acq is also used in other modalities so description should be generic.

_mod- change relates to a discussion of _mod destiny in

https://github.com/bids-standard/bids-2-devel/issues/70

and to accent that it is not really "modality" term (e.g. mri, eeg, ...) we use generally through out BIDS specification.

Some might argue about use of "contrast" to describe T1w etc but we do use "contrast" to that extent in other places already AFAIK.

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 87.93%. Comparing base (943c20e) to head (ff1d55b).

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #1778 +/- ## ======================================= Coverage 87.93% 87.93% ======================================= Files 16 16 Lines 1351 1351 ======================================= Hits 1188 1188 Misses 163 163 ```

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[yarikoptic]

I was wide off -- we did clarify modality definition in https://bids-specification.readthedocs.io/en/stable/common-principles.html#definitions and for better or for worse it was Ok and no need to bring in "contrast" here... I will adjust now

edit: nah -- better just close ;)