Fill up the missing docs for Index/MultiIndex

[itholic]

as commented in #1114 (comment),

i filled up the missing docs for Index and MultiIndex

[codecov-io]

Codecov Report

Merging #1123 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #1123   +/-   ##
=======================================
  Coverage   95.15%   95.15%           
=======================================
  Files          35       35           
  Lines        7016     7016           
=======================================
  Hits         6676     6676           
  Misses        340      340

---

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 0cbedc1...eb596f1. Read the comment docs.