docs: dt: Update overlay file extension

Building DTB overlays from .dts files is no longer supported.
Update the documentation to reflect this.

Fixes: 81d362732bac05f6 ("kbuild: Disallow DTB overlays to built from .dts named source files")
Signed-off-by: Geert Uytterhoeven <[e-mail]>
Acked-by: Andrew Davis <[e-mail]>
Reviewed-by: Yanteng Si <[e-mail]>
Link: https://lore.kernel.org/r/ebce4[e-mail]
Signed-off-by: Rob Herring (Arm) <[e-mail]>

Documentation/devicetree/overlay-notes.rst

···
 	};
     ---- foo.dts ---------------------------------------------------------------
 
-The overlay bar.dts,
+The overlay bar.dtso,
 ::
 
-    ---- bar.dts - overlay target location by label ----------------------------
+    ---- bar.dtso - overlay target location by label ---------------------------
 	/dts-v1/;
 	/plugin/;
 	&ocp {
···
 			... /* various properties and child nodes */
 		};
 	};
-    ---- bar.dts ---------------------------------------------------------------
+    ---- bar.dtso --------------------------------------------------------------
 
 when loaded (and resolved as described in [1]) should result in foo+bar.dts::
 
···
 location by label syntax is preferred because the overlay can be applied to
 any base DT containing the label, no matter where the label occurs in the DT.
 
-The above bar.dts example modified to use target path syntax is::
+The above bar.dtso example modified to use target path syntax is::
 
-    ---- bar.dts - overlay target location by explicit path --------------------
+    ---- bar.dtso - overlay target location by explicit path -------------------
 	/dts-v1/;
 	/plugin/;
 	&{/ocp} {
···
 			... /* various properties and child nodes */
 		}
 	};
-    ---- bar.dts ---------------------------------------------------------------
+    ---- bar.dtso --------------------------------------------------------------
 
 
 Overlay in-kernel API

Documentation/translations/zh_CN/devicetree/overlay-notes.rst

···
 	};
     ---- foo.dts ---------------------------------------------------------------
 
-覆盖bar.dts,
+覆盖bar.dtso,
 ::
 
-    ---- bar.dts - 按标签覆盖目标位置 ----------------------------
+    ---- bar.dtso - 按标签覆盖目标位置 ---------------------------
 	/dts-v1/;
 	/插件/;
 	&ocp {
···
 			... /* 各种属性和子节点 */
 		};
 	};
-    ---- bar.dts ---------------------------------------------------------------
+    ---- bar.dtso --------------------------------------------------------------
 
 当加载（并按照[1]中描述的方式解决）时，应该产生foo+bar.dts::
 
···
 DT中的适当位置。在这种情况下，可以提供目标路径。通过标签的目标位置的语法是比
 较好的，因为不管标签在DT中出现在哪里，覆盖都可以被应用到任何包含标签的基础DT上。
 
-上面的bar.dts例子被修改为使用目标路径语法，即为::
+上面的bar.dtso例子被修改为使用目标路径语法，即为::
 
-    ---- bar.dts - 通过明确的路径覆盖目标位置 --------------------
+    ---- bar.dtso - 通过明确的路径覆盖目标位置 -------------------
 	/dts-v1/;
 	/插件/;
 	&{/ocp} {
···
 			... /* 各种外围设备和子节点 */
 		}
 	};
-    ---- bar.dts ---------------------------------------------------------------
+    ---- bar.dtso --------------------------------------------------------------
 
 
 内核中关于覆盖的API