doc string updates and change of default parameter

Some minor docstring updates
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* corrected grammatical errors
* added some information about the `percentile` value (range)  in both `image.high_variance_confounds` and `signal.high_variance_confounds`, since current documentation is contradictory

Minor update on default values
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* default value of 2 percent of highest variance voxels for `percentile` in both `image.high_variance_confounds` and `signal.high_variance_confounds`, as to comply with the recommendations in the ComCor paper