add quaternion mul* docs

doc/en/scripting/builtins/libquat.md

@@ -2,7 +2,10 @@
 
 Quaternions manipulation library.
 
-## Quaternion from matrix - *mat4.from_quat(...)*
+Quaternions can be created as a table of 4 numbers:
+`{w, x, y, z}`
+
+## Quaternion from matrix - *quat.from_mat4(...)*
 
 ```lua
 -- creates a quaternion based on the rotation matrix
@@ -12,6 +15,38 @@ quat.from_mat4(m: matrix)
 quat.from_mat4(m: matrix, dst: quat)
 ```
 
+## Quaternion of Euler angles - *quat.from_euler(...)*
+
+```lua
+-- creates quaternion from euler angles passed as XYZ (pitch, yaw, roll)
+-- angles in degrees
+quat.from_euler(euler: vec3) -> quat
+
+-- writes quaternion from euler angles passed as XYZ (pitch, yaw, roll) to dst
+-- angles in degrees
+quat.from_euler(euler: vec3, dst: quat)
+```
+
+## Quaternion composition - quat.mul(...)
+
+```lua
+-- multiplies quaternions
+quat.mul(a: quat, b: quat) -> quat
+
+-- writes a quaternions multiplication result to dst
+quat.mul(a: quat, b: quat, dst: quat)
+```
+
+## 3D vector rotation - quat.mul_vec3(...)
+
+```lua
+-- rotates vector by quaternion
+quat.mul_vec3(a: quat, b: vec3) -> vec3
+
+-- writes rotated vector by quaternion to dst
+quat.mul_vec3(a: quat, b: vec3, dst: vec3)
+```
+
 ## Spherical linear interpolation - *quat.slerp(...)*
 
 The interpolation always take the short path and the rotation is performed at constant speed.