Update the RNA and DNA documentation for two bone matrices:
- `PoseBone.matrix_channel` (`bPoseChannel::chan_mat` in DNA) contains
the evaluated loc/rot/scale channels, including constraints and drivers.
- `PoseBone.matrix` (`bPoseChannel::pose_mat` in DNA) contains the same
transform, but then expressed in the armature object space.
No functional changes, just clarifications in comments / tooltips.
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