Cleanup: document ANIM_animdata_get_context side-effects

Document a (for me unexpected) side-effect of `ANIM_animdata_get_context`.

No functional changes.

source/blender/editors/include/ED_anim_api.hh

@@ -489,6 +489,9 @@ bool ANIM_animdata_get_context(const bContext *C, bAnimContext *ac);
  * - AnimContext to write to is provided as pointer to var on stack so that we don't have
  *   allocation/freeing costs (which are not that avoidable with channels).
  * \return whether the operation was successful.
+ *
+ * \note This may also update the space data. For example, `SpaceAction::action`
+ * is set to the currently active object's Action.
  */
 bool ANIM_animdata_context_getdata(bAnimContext *ac);