Improve description of storage root extensions

[zimeon]

We currently describe Storage Root extensions by reference to Object extensions. This is fine in terms of what the structure is and an error in the storage root extensions structure should result in E086. However, there is no warning code defined for use of an unregistered extension in a storage root. The code W013 is used for object extensions but can't simply be used again for Storage Root extensions.

I think the best solution is simply to write out the structure requirement again for Storage Root extensions, commenting that this is the same as Object extensions. We can keep the error code E086 and the define a new warning code for use of an unregistered extension.

[rosy1280]

Editors agreed with @zimeon 's recommendation. @awoods will create a pull request for this ticket.

[awoods]

Two potential resolutions:

1. Replicate specification of Object extension into Storage Root extensi… #583
2. Clarify that extension directory warning applies to Objects and the S… #582

I lean towards the second (582)

[zimeon]

Editors agree that the replication method provides a cleaner solution for readers and it is helpful to have separate error codes for the object and storage root cases.

[zimeon]

The issue @pwinckles raised #583 (review) about the proposed E113 being unenforceable overlaps with #565. I think we could move forward with this issue by simply removing the text from the E113 sentence onward in #583, and rely on a resolution of #565 to address the notion of what constitutes a registered extension name.