Fixes and improvements to docs

[oskooi]

Two fixes and improvements to the documentation based on recent discussions:

1. In Tutorial/Cylindrical Coordinates/Nonaxisymmetric Dipole Sources, the formulas for the radial Poynting flux $\vec{E} \times \vec{H}^*$ were missing the term $\cdot \hat{r}$. See Latex renderings below.
2. In FAQ/How do I compute the effective index of an eigenmode of a lossy waveguide?, it is useful to also describe how to compute the effective index of a lossless system as pointed out by @stevengj in #701 (comment).

Also includes a couple of spelling corrections.

master branch

this PR

[codecov-commenter]

Codecov Report

Merging #2500 (0a641c4) into master (d0d81bb) will increase coverage by 0.11%. The diff coverage is n/a.

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@@            Coverage Diff             @@
##           master    #2500      +/-   ##
==========================================
+ Coverage   73.82%   73.94%   +0.11%     
==========================================
  Files          17       18       +1     
  Lines        5269     5293      +24     
==========================================
+ Hits         3890     3914      +24     
  Misses       1379     1379              

see 2 files with indirect coverage changes