Expression calculator: clarify documentation for x_at() and y_at()

(cherry picked from commit 5256611)

Author: AlisterH

resources/function_help/json/$x_at

@@ -2,7 +2,7 @@
   "name": "$x_at",
   "type": "function",
   "description": "Retrieves a x coordinate of the current feature's geometry.",
-  "arguments": [ {"arg":"i","description":"index of point of a line (indices start at 0; negative values apply from the last index)"}],
+  "arguments": [ {"arg":"i","description":"index of point of a line (indices start at 0; negative values apply from the last index, starting at -1)"}],
   "examples": [ { "expression":"$x_at(1)", "returns":"5"}
   ]
 }

resources/function_help/json/$y_at

@@ -2,7 +2,7 @@
   "name": "$y_at",
   "type": "function",
   "description": "Retrieves a y coordinate of the current feature's geometry.",
-  "arguments": [ {"arg":"i","description":"index of point of a line (indices start at 0; negative values apply from the last index)"}],
+  "arguments": [ {"arg":"i","description":"index of point of a line (indices start at 0; negative values apply from the last index, starting at -1)"}],
   "examples": [ { "expression":"$y_at(1)", "returns":"2"}
   ]
 }