docs/CODING_STYLE: document that we nowadays prefer (const char*) for func ret type

Addresses https://github.com/systemd/systemd/pull/33567#discussion_r1662818225

(cherry picked from commit 4b7249111a4c1d366f476bdbd6e03f7893eb9d42)
(cherry picked from commit 3c91ea49d249cda6e1fa84b53a42dee6d83339da)

docs/CODING_STYLE.md

@@ -54,6 +54,18 @@ SPDX-License-Identifier: LGPL-2.1-or-later
   }
   ```
 
+- Function return types should be seen/written as whole, i.e. write this:
+
+  ```c
+  const char* foo(const char *input);
+  ```
+
+  instead of this:
+
+  ```c
+  const char *foo(const char *input);
+  ```
+
 - Single-line `if` blocks should not be enclosed in `{}`. Write this:
 
   ```c