Improve aqi description

[chrisgorgo]

By @nicholst

Secondly, here's an epsilon improvement to the docs. For aqi you just say:
AFNI’s quality index (aqi) - Mean quality index as computed by AFNI’s 3dTqual.
but it'd be much nicer if you just say what it is:
AFNI’s quality index (aqi) - Mean quality index as computed by AFNI’s 3dTqual; for each volume, it is one minus the Spearman's (rank) correlation of that volume with the median volume.

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