setenv documentation in elisp manual outdated

[Roland Winkler]

The setenv command now takes two new optional arguments UNSET and
SUBSTITUTE-ENV-VARS, which are not discussed in the elisp manual.
Actually, I wanted to look into the elisp manual because from the
doc string of setenv it was not clear to me what the difference was
between a nil value of VALUE and a value of t for UNSET. When does
one need the optional arg UNSET because nil for VALUE is not doing
the job?

Thanks,

Roland