Thanks to Stephen Hemminger for pointing out how obsolete modules.txt is.

modules.txt contains mainly ancient information which is replicated
in the kconfig help message, README, makefile.txt or the modprobe manual
page.  The only part which is not covered elsewhere is the "building
external modules" which is still being debated (and belongs under the
kbuild docs).  kmod.txt reference removed from index, too.