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I haven't used Doxygen that much, but after checking [the documentation](https://www.doxygen.nl/manual/index.html) and [looking around online](https://linux.m2osw.com/doxygen-does-not-generate-documentation-my-c-functions-or-any-global-function), it looks like the reason this might be happening is because the SHOW_USED_FILES setting which is used to show the list of files at the bottom of a page of documentation appears to be limited to the documentation of classes and structs.

In other words, if an `enum` is not particularly in a class, and if it's not somehow a `struct`, the outputted documentation page should not include this list of source files.

I tried out a test using C language files with an `enum` and a `struct`. When I tried just using an `enum` definition in my header file, I did not see the `The documentation for this struct was generated from the following file:` line from the bottom of the generated Doxygen page. When I included a struct definition in my header file, I finally noticed that the resulting page was under the `Classes` menu on the documentation generated by Doxygen. After clicking on the `Classes` menu and then clicking on the name of the `struct` I used, I was able to see the `The documentation for this `struct` was generated from the following file:` line at the bottom of the generated documentation.

Thus, from what I read in the documentation, it looks like a plain `enum` file is not meant to show the `The documentation for this struct was generated from the following file:` line. If your `enum` could somehow be a part of a class, however, like in [this example from the documentation](https://www.doxygen.nl/manual/examples/afterdoc/html/class_afterdoc___test.html), it should work!

For reference, here are the C files I used for my tests:

**q12.h**
```
#include <stdio.h>
#include <string.h>

// ref: https://docs.microsoft.com/en-us/cpp/c-language
//   /c-enumeration-declarations?view=msvc-160
enum COLOR
{
    Red, /*!< Enum value red. */
    Blue, /*!< Enum value blue. */
    Yellow /*!< Enum value yellow. */
};

// ref: https://www.tutorialspoint.com/cprogramming/c_structures.htm
struct Colors {
    char name[30]; /*!< Struct name. */
};
```

**q12.c**
```
#include <stdio.h>
#include "q12.h"

int main()
{
    // ref: https://docs.microsoft.com/en-us/cpp/c-language
    //   /c-enumeration-declarations?view=msvc-160
    enum COLOR primary = Red;
    // ref: https://www.tutorialspoint.com/cprogramming/c_structures.htm
    struct Colors Color1;
    strcpy(Color1.name, "Red");
    printf("Hello, world!\n");
    printf("Printing color: %d\n", primary);
    return 0;
}
```