doc and emcee_example fixes

[newville]

These are doc and example fixes only, improving the description for using minimizer.emcee() and so partly addressing #601. That is, the docs now sort of explain that ``minimize.emcee()is not really doing a fit, but instead "only" exploring parameter space. It still might be nice to move theemcee` code to be with `conf_interval`, but that is too big of a change for today.

Since this is doc only, my intention is to merge this (once or when tests pass) and then release as 1.0.
If there are any errors, omissions, or inaccuracies in the docs they can be fixed at any time...

[codecov]

Codecov Report

Merging #614 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #614   +/-   ##
=======================================
  Coverage   91.03%   91.03%           
=======================================
  Files          11       11           
  Lines        3289     3289           
=======================================
  Hits         2994     2994           
  Misses        295      295

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