feat(python): Document disabled-by-default middleware spans (#15923)

The `middleware_spans` option was missing from the FastAPI documentation.
Adjust working to clarify that the option gates creating one span per middleware layer.

Documents getsentry/sentry-python#5219 and getsentry/sentry-python#5224.

Author: alexander-alderman-webb

docs/platforms/python/integrations/django/index.mdx

@@ -151,7 +151,7 @@ You can pass the following keyword arguments to `DjangoIntegration()`:
 
 - `middleware_spans`:
 
-  Create spans and track performance of all middleware in your Django project. Set to `False` to disable.
+  Create spans and track performance of all middleware layers in your Django project. Set to `True` to enable.
 
   The default is `False`.
 

docs/platforms/python/integrations/fastapi/index.mdx

@@ -161,6 +161,12 @@ You can pass the following keyword arguments to `StarletteIntegration()` and `Fa
 
   The default is `{*range(500, 600)}`, meaning that all 5xx status codes are reported to Sentry.
 
+- `middleware_spans`:
+
+  Create spans and track performance of all middleware layers in your FastAPI project. Set to `True` to enable.
+
+  The default is `False`.
+
 - `http_methods_to_capture`:
 
   A tuple containing all the HTTP methods that should create a transaction in Sentry.

docs/platforms/python/integrations/starlette/index.mdx

@@ -108,9 +108,9 @@ You can pass the following keyword arguments to `StarletteIntegration()`:
 
 - `middleware_spans`:
 
-  Create spans and track performance of all middleware in your Starlette project. Set to `False` to disable.
+  Create spans and track performance of all middleware layers in your Starlette project. Set to `True` to enable.
 
-  The default is `True`.
+  The default is `False`.
 
 - `http_methods_to_capture`: