Update converters.rst
merged branch mmoreram/patch-1 (PR #228)

This PR was submitted for the master branch but it was merged into the 2.2 branch instead (closes #228).

Discussion
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Update converters.rst

Documentation is not clear, so can introduce errors in implementation.
Now, explain what it means both the key and the value.

Commits
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8fb8e36 Update converters.rst
tweaked docs