Easy to forget to copy changes to newer sdf/1.X folders when merging forward

## Description

I have noticed several pull requests that add features or fix documentation to a version of the SDFormat specification contained in the `sdf/1.X` folders that doesn't get propagated to newer folders when merging forward. For example, camera type aliases were added to the documentation of `sdf/1.6/camera.sdf` and `sdf/1.7/camera.sdf` in #514, but were not added to `sdf/1.8/camera.sdf` when merged forward to `sdf11` in #520. This is easy to forget and not documented as part of our forward porting policy. We should determine an improved process and document it. If possible, we could try to create automated testing to remind us about this, though I'm not sure how.