Add documentation for string.templatelib.convert()

Author: davepeck

Doc/library/string.rst

@@ -265,6 +265,8 @@ Some simple format string examples::
    "Weight in tons {0.weight}"       # 'weight' attribute of first positional arg
    "Units destroyed: {players[0]}"   # First element of keyword argument 'players'.
 
+.. _formatstrings-conversion:
+
 The *conversion* field causes a type coercion before formatting.  Normally, the
 job of formatting a value is done by the :meth:`~object.__format__` method of the value
 itself.  However, in some cases it is desirable to force a type to be formatted

Doc/library/string.templatelib.rst

@@ -1,14 +1,13 @@
-:mod:`!string.templatelib` --- Templates and interpolations for t-strings
-=========================================================================
+:mod:`!string.templatelib` --- Support for template string literals
+===================================================================
 
 .. module:: string.templatelib
-   :synopsis: Support for t-string literals.
+   :synopsis: Support for template string literals.
 
 **Source code:** :source:`Lib/string/templatelib.py`
 
 --------------
 
-
 .. seealso::
 
    * :ref:`Format strings <f-strings>`
@@ -294,3 +293,21 @@ reassigned.
    ...         print(value, expression, conversion, format_spec)
    ...
    3.0 1 + 2 None .2f
+
+
+Helper functions
+----------------
+
+.. function:: convert(obj, /, conversion)
+
+   Applies formatted string literal :ref:`conversion <formatstrings-conversion>`
+   semantics to the given object *obj*.
+   This is frequently useful for custom template string processing logic.
+
+   Three conversion flags are currently supported:
+
+   * ``'!s'`` which calls :func:`str` on the value,
+   * ``'!r'`` which calls :func:`repr`, and
+   * ``'!a'`` which calls :func:`ascii`.
+
+   If the conversion flag is ``None``, *obj* is returned unchanged.