{every,even,odd}{head,foot}ing customization var doc

[Karl Berry]

According to the "Customization Variables for @-Commands" node
(https://www.gnu.org/software/texinfo/manual/texinfo/texinfo.html#Customization-Variables-for-_0040_002dCommands)
there are customization variables for the various head/foot @-commands.

But I guess the implication of the text there:
  Setting such a customization variable to a value @samp{foo} is similar
  to executing @code{@@@var{cmd} foo}.  
is that these are no-ops.

That is the behavior I observe, anyway, e.g., after
  makeinfo -c everyheading='lft@|ctr@|rt' --html hello.texi
the given strings don't show up anywhere in the output.
Texinfo/Common.pm says they are "only relevant in TeX, and special".

Furthermore, it seems that specifying these config values even with
--pdf has no effect, e.g., the -c makes no difference to the output here:
  makeinfo -c everyheading='lft@|ctr@|rt' --pdf hello.texi
(And I wouldn't expect this to work, since it would take a lot of pain
to pass the setting to TeX.)

Thus, I suggest the documentation would be clearer if it said these are
parsed but ignored by makeinfo, so people don't think they are going to
control, e.g., the HTML head/foot with them. Or, even simpler, don't
even bother to mention them, unless they have some purpose to tp that I
am not seeing.

I surmise the same no-op-ness is true of others in that list, e.g.,
@setchapternewpage, but I didn't do more experiments. --best, karl.