pythonGH-126748: amend configure.rst description for the 'build_wasm'…

… make target (python#126687)

Clarified the documentation around how the build_wasm target is selected.

Doc/using/configure.rst

@@ -1088,7 +1088,8 @@ CPython project) this is usually the ``all`` target. The
 all`` will build. The three choices are:
 
 * ``profile-opt`` (configured with ``--enable-optimizations``)
-* ``build_wasm`` (configured with ``--with-emscripten-target``)
+* ``build_wasm`` (chosen if the host platform matches ``wasm32-wasi*`` or
+  ``wasm32-emscripten``)
 * ``build_all`` (configured without explicitly using either of the others)
 
 Depending on the most recent source file changes, Make will rebuild