(core) Update documentation of CURRENT_CONVERSION function

Summary: As requested by Anais in https://github.com/dsagal/grist-help/pull/132

Test Plan: Only a change to doc-comment (from which help docs are built).

Reviewers: alexmojaki

Reviewed By: alexmojaki

Differential Revision: https://phab.getgrist.com/D3267

sandbox/grist/functions/info.py

@@ -502,8 +502,8 @@ def N(value):
 
 def CURRENT_CONVERSION(rec):
   """
-  Special function used only when changing the type of a column.
-  Doesn't work in normal formulas.
+  Internal function used by Grist during column type conversions. Not available for use in
+  formulas.
   """
   return rec.gristHelper_Converted