Add footnote on file_operations API changes

The main text's 'struct file_operations' example is intentionally kept
at the v5.4 LTS version to ensure compatibility with the guide's code.

To bridge the gap with modern kernels, a new footnote is introduced. It
summarizes the evolution of the struct up to Linux v6.12, documenting
key additions, removals, and modifications to its members. This
provides crucial context for readers working with newer kernel versions.

Co-authored-by: EricccTaiwan <yphbchou0911@gmail.com>

Author: Jordymalone

lkmpg.tex

@@ -956,6 +956,9 @@ \subsection{The file\_operations Structure}
 Some operations are not implemented by a driver.
 For example, a driver that handles a video card will not need to read from a directory structure.
 The corresponding entries in the \cpp|file_operations| structure should be set to \cpp|NULL|.
+\footnote{
+As of Linux kernel 6.12, several member fields have been added, removed, or had their prototypes changed. For example, additions include \texttt{fop\_flags}, \texttt{splice\_eof}, and \texttt{uring\_cmd}; removals include \texttt{iterate} and \texttt{sendpage}; and the prototype for \texttt{iopoll} was modified.
+}
 
 There is a gcc extension that makes assigning to this structure more convenient.
 You will see it in modern drivers, and may catch you by surprise.