Add dual in RelativeEntropyCone docstring

[blegat]

It can be obtained from the bridge to exponential cone.
[I 0]^(-1) * RelEntr = [A B]^(-1) [Exp1 x ... x Expn x R+]
so
[I; 0] * RelEntr* = [A'; B'] * [Exp1* x ... x Expn* x R+]
B' * R+ = 0 says that the u_i are all equal.
Then A' * [Exp1* x ... x Expn*] gives the dual exponential cone constraints with the same u_i

[codecov-io]

Codecov Report

Merging #1018 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #1018   +/-   ##
=======================================
  Coverage   95.19%   95.19%           
=======================================
  Files         100      100           
  Lines       11279    11279           
=======================================
  Hits        10737    10737           
  Misses        542      542

Impacted Files	Coverage Δ
src/sets.jl	95.28% <ø> (ø)	⬆️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update e7331c4...6d57269. Read the comment docs.

[chriscoey]

-u \\exp (v_i/u) \\le \\exp(1) w_i is equivalent to v_i \\le u (\\log(w_i/-u) + 1), not sure if worth noting both.