libkmod: document KMOD_INDEX_MODULES_BUILTIN_ALIAS in kmod_dump_index()

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 <gladkov.alexey@gmail.com>
Fixes: b866b21 ("Lookup aliases in the modules.builtin.modinfo")
Signed-off-by: Emil Velikov <emil.l.velikov@gmail.com>
Reviewed-by: Lucas De Marchi <lucas.demarchi@intel.com>
Link: https://lore.kernel.org/r/20240719-abi-fixes-v1-1-1e6d99a2846b@gmail.com