remove warning and add it as doc string (#4240)

nilearn/experimental/__init__.py

@@ -1,21 +1,18 @@
 """The :mod:`nilearn.experimental` module provides importable modules that \
 enable the use of experimental features.
 
-The features and estimators that are experimental aren't subject to
-deprecation cycles. Use them at your own risks!
-"""
+.. warning::
 
-import warnings
+    All features in the ``nilearn.experimental`` module
+    are experimental and subject to change.
 
-warnings.warn(
-    "\n\n\nAll features in the nilearn.experimental module are "
-    "experimental and subject to change. They are included "
-    "in the nilearn package to gather early feedback from users "
-    "about prototypes of new features. "
-    "Changes may break backwards compatibility without prior "
-    "notice or a deprecation cycle. Moreover, some features may "
-    "be incomplete or may have been tested less thoroughly than "
-    "the rest of the library.\n\n"
-)
+    They are included in the nilearn package
+    to gather early feedback from users about prototypes of new features.
+    Changes may break backwards compatibility
+    without prior notice or a deprecation cycle.
+    Moreover, some features may be incomplete
+    or may have been tested less thoroughly than the rest of the library.
 
+    Use those features at your own risks!
+"""
 __all__ = ["surface"]