Document the X option and env var for controlling thread-local bytecode

Add documentation for `-X tlbc` and `PYTHON_TLBC`.

Author: mpage

Doc/using/cmdline.rst

@@ -669,6 +669,13 @@ Miscellaneous options
 
      .. versionadded:: 3.14
 
+   * :samp:`-X tlbc={0,1}` enables (1, the default) or disables (0) thread-local
+     bytecode in builds configured with :option:`--disable-gil`.  When disabled,
+     this also disables specialization in the tier 1 interpreter.  See also
+     :envvar:`PYTHON_TLBC`.
+
+     .. versionadded:: 3.14
+
    It also allows passing arbitrary values and retrieving them through the
    :data:`sys._xoptions` dictionary.
 
@@ -1302,6 +1309,16 @@ conflict.
 
    .. versionadded:: 3.13
 
+.. envvar:: PYTHON_TLBC
+
+   If set to ``1`` enables thread-local bytecode. If set to ``0`` thread-local
+   bytecode (and specialization in the tier 1 interpreter) is disabled. Only
+   applies to builds configured with :option:`--disable-gil`.
+
+   See also the :option:`-X tlbc <-X>` command-line option
+
+   .. versionadded:: 3.14
+
 Debug-mode variables
 ~~~~~~~~~~~~~~~~~~~~