📝 Clarify valid motion_estimate_filter settings

[shnizzedy]

Fixes

Fixes #1425 by @shnizzedy

Description

• Updates, attempts to clarify, the comments in the default pipeline config based on i'm confused by `motion_estimate_filter` configuration comments #1425 (reply in thread).
• Updates the schema to allow parameters to be set but unused based on i'm confused by `motion_estimate_filter` configuration comments #1425 (reply in thread), so any of the rows in the following table are valid settings:

run	filter_type	breathing_rate_min	breathing_rate_max	center_frequency	filter_bandwidth	lowpass_cutoff
off	doesn't matter	doesn't matter	doesn't matter	doesn't matter	doesn't matter	doesn't matter
on	notch	required	required	doesn't matter	doesn't matter	doesn't matter
on	notch	None	None	required	required	doesn't matter
on	lowpass	required	doesn't matter	doesn't matter	doesn't matter	doesn't matter
on	lowpass	None	doesn't matter	doesn't matter	doesn't matter	required

Prior to this change, the schema validation would raise an error on load if both motion estimate filter design options were provided (the alternative listed in #1430:

We can also throw an error, but this may be too disruptive.

)

Checklist

• My pull request has a descriptive title (not a vague title like Update index.md).
• My pull request targets the develop_v1.8_convergence branch of the repository.
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• I added tests for the changes I made (if applicable).
• I updated the changelog.
• I added or updated documentation (FCP-INDI/fcp-indi.github.io@fce06bf).
• I tried running the project locally and verified that there are no
  visible errors.

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