DOC: document calculation of precision error

Part of the discussion of the rotation matrix thresholds on githab at:
https://github.com/nipy/nibabel/pull/156
BF: relax rotation matrix threshold

The threshold was probably too low for lower precision dicoms.

As discussed in https://github.com/nipy/nibabel/pull/156
Merge pull request #167 from matthew-brett/relax-rotation-threshold

MRG: Relax rotation threshold

Relax threshold for orthogonality of rotation matrix to deal with low-precision values in DICOM Image Orient Patient field.