Fix #107032: API Document: matrix_channel (PoseBone) description incorrect

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.

source/blender/makesdna/DNA_action_types.h

@@ -290,13 +290,14 @@ typedef struct bPoseChannel {
   char _pad[2];
 
   /**
-   * Matrix result of location/rotation/scale components & constraints.
+   * Matrix result of location/rotation/scale components, and evaluation of
+   * animation data and constraints.
+   *
    * This is the dynamic component of `pose_mat` (without #Bone.arm_mat).
    */
   float chan_mat[4][4];
   /**
-   * Constraints accumulate here. in the end, `pose_mat = bone->arm_mat * chan_mat`
-   * this matrix is object space.
+   * Channel matrix in the armature object space, i.e. `pose_mat = bone->arm_mat * chan_mat`.
    */
   float pose_mat[4][4];
   /** For display, pose_mat with bone length applied. */

source/blender/makesrna/intern/rna_pose.c

@@ -1134,7 +1134,10 @@ static void rna_def_pose_channel(BlenderRNA *brna)
   RNA_def_property_float_sdna(prop, NULL, "chan_mat");
   RNA_def_property_multi_array(prop, 2, rna_matrix_dimsize_4x4);
   RNA_def_property_clear_flag(prop, PROP_EDITABLE);
-  RNA_def_property_ui_text(prop, "Channel Matrix", "4x4 matrix, before constraints");
+  RNA_def_property_ui_text(prop,
+                           "Channel Matrix",
+                           "4x4 matrix of the bone's location/rotation/scale channels (including "
+                           "animation and drivers) and the effect of bone constraints");
 
   /* writable because it touches loc/scale/rot directly */
   prop = RNA_def_property(srna, "matrix_basis", PROP_FLOAT, PROP_MATRIX);
@@ -1156,7 +1159,7 @@ static void rna_def_pose_channel(BlenderRNA *brna)
   RNA_def_property_ui_text(
       prop,
       "Pose Matrix",
-      "Final 4x4 matrix after constraints and drivers are applied (object space)");
+      "Final 4x4 matrix after constraints and drivers are applied, in the armature object space");
   RNA_def_property_update(prop, NC_OBJECT | ND_POSE, "rna_Pose_update");
 
   /* Head/Tail Coordinates (in Pose Space) - Automatically calculated... */