gh-129438: Update --enable-experimental-jit section with install requirements (#129450)

Add a note to the `JIT` docs that building CPython with `JIT` requires Python 3.11 or newer.

Co-authored-by: Brandt Bucher <[email protected]>

Author: StanFromIreland

Doc/using/configure.rst

@@ -311,6 +311,10 @@ General Options
 
    By convention, ``--enable-experimental-jit`` is a shorthand for ``--enable-experimental-jit=yes``.
 
+   .. note::
+
+      When building CPython with JIT enabled, ensure that your system has Python 3.11 or later installed.
+
    .. versionadded:: 3.13
 
 .. option:: PKG_CONFIG

Tools/jit/README.md

@@ -3,6 +3,8 @@ The JIT Compiler
 
 This version of CPython can be built with an experimental just-in-time compiler[^pep-744]. While most everything you already know about building and using CPython is unchanged, you will probably need to install a compatible version of LLVM first.
 
+Python 3.11 or newer is required to build the JIT.
+
 ## Installing LLVM
 
 The JIT compiler does not require end users to install any third-party dependencies, but part of it must be *built* using LLVM[^why-llvm]. You are *not* required to build the rest of CPython using LLVM, or even the same version of LLVM (in fact, this is uncommon).
@@ -54,7 +56,7 @@ For `PCbuild`-based builds, pass the new `--experimental-jit` option to `build.b
 
 For all other builds, pass the new `--enable-experimental-jit` option to `configure`.
 
-Otherwise, just configure and build as you normally would. Cross-compiling "just works", since the JIT is built for the host platform. 
+Otherwise, just configure and build as you normally would. Cross-compiling "just works", since the JIT is built for the host platform.
 
 The JIT can also be enabled or disabled using the `PYTHON_JIT` environment variable, even on builds where it is enabled or disabled by default. More details about configuring CPython with the JIT and optional values for `--enable-experimental-jit` can be found [here](https://docs.python.org/dev/whatsnew/3.13.html#experimental-jit-compiler).