How to save changes to patch files for kernel headers in `include/linux` using `diff`?

I am trying to hack the Linux kernel and added/modified some header files in include/linux. However, when I generate patch file using diff like:

bash
diff -uprN -X linux/Documentation/dontdiff \
        linux-5.15 linux-srctree > patch.p1

I find changes under include/linux not included in patch.p1. I then look into dontdiff file and find the following lines (around line 158), which means it excludes include/linux from the patch file.

*lex.*.c
linux
logo_*.c

So why does dontdiff include linux? How to save changes to patch files for headers in include/linux? Thanks a lot.