doc: add *-inl.h include rule to C++ style guide

PR-URL: #16548
Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>
Reviewed-By: Gireesh Punathil <gpunathi@in.ibm.com>
Reviewed-By: Daniel Bevenius <daniel.bevenius@gmail.com>
Reviewed-By: Gibson Fahnestock <gibfahn@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>

CPP_STYLE_GUIDE.md

@@ -127,6 +127,21 @@ class FancyContainer {
 
 What it says in the title.
 
+## Do not include `*.h` if `*-inl.h` has already been included
+
+Do
+
+```cpp
+#include "util-inl.h"  // already includes util.h
+```
+
+instead of
+
+```cpp
+#include "util.h"
+#include "util-inl.h"
+```
+
 ## Avoid throwing JavaScript errors in nested C++ methods
 
 If you need to throw JavaScript errors from a C++ binding method, try to do it