Tidy up UI node docs (bevyengine#10189)

# Objective

While reviewing bevyengine#10187 I noticed some other mistakes in the UI node
docs.

## Solution

I did a quick proofreading pass and fixed a few things. And of course,
the typo from that other PR.

## Notes

I occasionally insert a period to make a section of doc self-consistent
but didn't go one way or the other on all periods in the file.

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Co-authored-by: Noah <[email protected]>