gh-148588: Fix __lazy_modules__ use-case description

Replace 'migrating a large existing codebase' with the more accurate
use case: supporting Python <3.15 while leveraging lazy imports on 3.15+.

Author: johnslavik

Doc/reference/simple_stmts.rst

@@ -937,8 +937,9 @@ sequence of absolute module name strings.  Any regular (non-``lazy``)
 :keyword:`lazy` keyword had been used.
 
 This provides a way to enable lazy loading for specific dependencies without
-changing individual ``import`` statements — useful when migrating existing
-code or when the imports are generated programmatically::
+changing individual ``import`` statements — useful when supporting Python
+versions older than 3.15 while leveraging lazy imports on 3.15+, or when
+the imports are generated programmatically::
 
    __lazy_modules__ = ["json", "pathlib"]
 

Doc/whatsnew/3.15.rst

@@ -183,7 +183,8 @@ function, class body, or ``try``/``except``/``finally`` block raises a
 raise :exc:`SyntaxError`).
 
 For code that cannot use the ``lazy`` keyword directly — for example, when
-migrating a large existing codebase — a module can define
+supporting Python versions older than 3.15 while still leveraging lazy
+imports on 3.15+ — a module can define
 :attr:`~module.__lazy_modules__` as a sequence of absolute module name
 strings.  Regular ``import`` statements for those modules are then treated
 as lazy, with the same semantics as the ``lazy`` keyword::