add turboslit description

src/dodal/devices/turbo_slit.py

@@ -6,11 +6,15 @@ class TurboSlit(Device):
     """
     This collection of motors coordinates time resolved XAS experiments.
     It selects a beam out of the polychromatic fan.
+    There is a 0.1 degrees beam that can move along a 90-ish degree arc.
+    A turboslit device moves after the beam, so that the gap aligns with the beam. 
+    The turboslit gap can be made smaller to increase energy resolution.
+    The xfine motor can move the slit in x direction at high frequencies for different scans.
     These slits can be scanned continously or in step mode.
     The relationship between the three motors is as follows:
-        - gap provides energy resolution
-        - xfine selects the energy
-        - arc - ???
+        - arc - position of the middle of the gap (coarse/ macro) extension 
+        - gap - width in mm, provides energy resolution
+        - xfine selects the energy as part of the high frequency scan
     """
 
     def __init__(self, prefix: str, name: str):