add a note to clarify up vector restriction for glm_lookat and glm_look

docs/source/cam.rst

@@ -167,11 +167,13 @@ Functions documentation
 
     | set up view matrix
 
+    **NOTE:** The UP vector must not be parallel to the line of sight from the eye point to the reference point.
+
     Parameters:
-    | *[in]*  **eye**     eye vector
-    | *[in]*  **center**  center vector
-    | *[in]*  **up**      up vector
-    | *[out]* **dest**    result matrix
+      | *[in]*  **eye**     eye vector
+      | *[in]*  **center**  center vector
+      | *[in]*  **up**      up vector
+      | *[out]* **dest**    result matrix
 
 .. c:function:: void  glm_look(vec3 eye, vec3 dir, vec3 up, mat4 dest)
 
@@ -181,6 +183,8 @@ Functions documentation
     target self then this might be useful. Because you need to get target
     from direction.
 
+    **NOTE:** The UP vector must not be parallel to the line of sight from the eye point to the reference point.
+
     Parameters:
       | *[in]*  **eye**     eye vector
       | *[in]*  **center**  direction vector

include/cglm/cam.h

@@ -321,6 +321,9 @@ glm_perspective_resize(float aspect,
 /*!
  * @brief set up view matrix
  *
+ * NOTE: The UP vector must not be parallel to the line of sight from
+ *       the eye point to the reference point
+ *
  * @param[in]  eye    eye vector
  * @param[in]  center center vector
  * @param[in]  up     up vector
@@ -364,6 +367,9 @@ glm_lookat(vec3 eye,
  * convenient wrapper for lookat: if you only have direction not target self
  * then this might be useful. Because you need to get target from direction.
  *
+ * NOTE: The UP vector must not be parallel to the line of sight from
+ *       the eye point to the reference point
+ *
  * @param[in]  eye    eye vector
  * @param[in]  dir    direction vector
  * @param[in]  up     up vector