erasure-code: document the ruleset-root profile parameter

If unspecified it is ruleset-root=default and will translate into

   take default

when a ruleset is created for an erasure-code pool.

Signed-off-by: Loic Dachary <loic@dachary.org>
Merge pull request #1650 from dachary/wip-erasure-code-doc

erasure-code: document the ruleset-root profile parameter

Reviewed-by: Mark Nelson <mark.nelson@inktank.com>