libkmod: document KMOD_INDEX_MODULES_BUILTIN_ALIAS in kmod_dump_index()

The extra enum was introduced a few years ago, although the (only)
function using it did not have it's documentation updated.

The commit itself has caused an ABI break with kmod v27, since the
KMOD_INDEX_MODULES_BUILTIN value has changed.

A search through Google, Github and Arch packages have shown one user of
kmod_dump_index() - the only API that uses the enum.

In that case and all others, no projects referencing the changed enum
were found. Although since recent and/or supported distros use kmod v27
or later we've decided to only update the documentation.

Cc: Alexey Gladkov <[email protected]>
Fixes: b866b21 ("Lookup aliases in the modules.builtin.modinfo")
Signed-off-by: Emil Velikov <[email protected]>
Reviewed-by: Lucas De Marchi <[email protected]>
Link: https://lore.kernel.org/r/[email protected]

libkmod/libkmod.c

@@ -971,6 +971,7 @@ KMOD_EXPORT void kmod_unload_resources(struct kmod_ctx *ctx)
  * KMOD_INDEX_MODULES_DEP: index of module dependencies;
  * KMOD_INDEX_MODULES_ALIAS: index of module aliases;
  * KMOD_INDEX_MODULES_SYMBOL: index of symbol aliases;
+ * KMOD_INDEX_MODULES_BUILTIN_ALIAS: index of builtin module aliases.
  * KMOD_INDEX_MODULES_BUILTIN: index of builtin module.
  * @fd: file descriptor to dump index to
  *