[3.14] Docs: avoid informal formulation in Doc/c-api/conversion.rst (GH-140898) (#140912)

Docs: avoid informal formulation in `Doc/c-api/conversion.rst` (GH-140898)
(cherry picked from commit 173cc53)

Co-authored-by: RayXu <[email protected]>

Author: miss-islington

Doc/c-api/conversion.rst

@@ -41,7 +41,7 @@ The return value (*rv*) for these functions should be interpreted as follows:
   ``rv + 1`` bytes would have been needed to succeed. ``str[size-1]`` is ``'\0'``
   in this case.
 
-* When ``rv < 0``, "something bad happened." ``str[size-1]`` is ``'\0'`` in
+* When ``rv < 0``, the output conversion failed and ``str[size-1]`` is ``'\0'`` in
   this case too, but the rest of *str* is undefined. The exact cause of the error
   depends on the underlying platform.