specification for <wind><linear velocity> does not specify units (sdf10-13)

## Environment
* Documentation for wind element in sdf10-13.
* http://sdformat.org/spec?ver=1.9&elem=world#wind_linear_velocity

## Description
* Expected behavior: I expect the specification for the `` element to indicate what units are being used. For example, specification for `` says "Temperature at sea level in **kelvins**" and `` says "Temperature gradient with respect to increasing altitude at sea level in units of K/m."
* Actual behavior: The description field of the `` element says "Linear velocity of the wind." but does not say what units are expected.

Would it be possible to update this description to include the units? Thank you!

Open the linked SDFormat 1.9 specification and inspect the `` entry alongside nearby unit-bearing entries. Confirm the project's expected unit convention, update that description to state the units, and verify the rendered specification shows the clarification.

Written by the indexing model from the issue text.