Indicate Mat3x3 and Mat4x4 constructors are column major (#4842)

### What

The comments were incorrect.

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rerun_cpp/src/rerun/datatypes/mat3x3_ext.cpp

@@ -27,7 +27,7 @@ namespace rerun {
                   columns[2].z(),
               } {}
 
-        /// Construct a new 3x3 matrix from a pointer to 9 floats (in row major order).
+        /// Construct a new 3x3 matrix from a pointer to 9 floats (in column major order).
         explicit Mat3x3(const float* elements)
             : flat_columns{
                   elements[0],

rerun_cpp/src/rerun/datatypes/mat4x4_ext.cpp

@@ -34,7 +34,7 @@ namespace rerun {
                   columns[3].w(),
               } {}
 
-        /// Construct a new 4x4 matrix from a pointer to 16 floats (in row major order).
+        /// Construct a new 4x4 matrix from a pointer to 16 floats (in column major order).
         explicit Mat4x4(const float* elements)
             : flat_columns{
                   elements[0],