Add docs for nnkHiddenStdConv (#16408)

Add it to devel branch this time. I hope this works.

doc/astspec.txt

@@ -1392,6 +1392,17 @@ Macro declaration
 Macros behave like templates, but ``nnkTemplateDef`` is replaced with
 ``nnkMacroDef``.
 
+Hidden Standard Conversion
+--------------------------
+
+.. code-block:: nim
+  var f: float = 1
+
+The type of "f" is ``float`` but the type of "1" is actually ``int``. Inserting 
+``int`` into a ``float`` is a type error. Nim inserts the ``nnkHiddenStdConv``  
+node around the ``nnkIntLit`` node so that the new node has the correct type of 
+``float``. This works for any auto converted nodes and makes the conversion 
+explicit.
 
 Special node kinds
 ==================