Nik Everett 59a43180a5 Reindex: Clean up docs around multi-index
We have an example in `reindex`'s docs about copying from many indices
at once. It doesn't work at the moment because we only allow a single
type per index. We didn't notice it in the docs tests because those
tests didn't copy any documents. This change:
1. Adds documents to the docs tests to fully exercise the snippet.
2. Fixes the example by moving all copied documents to the same type.
3. Moves the note about id collisions and expands on it because it is
even more likely than before.

Closes #35150
2018-11-05 12:52:11 -05:00
..