erasure-code: add definitions to the glossary

and fix a typo

Signed-off-by: Loic Dachary <loic@dachary.org>
Merge pull request #2039 from dachary/wip-erasure-code-doc

erasure-code: add definitions to the glossary

Reviewed-by: Florent Flament <florent.flament@cloudwatt.com>