Remove trailing whitespace from docs.

Trailing whitespace is widely considered a bad idea, so this commit removes it from all the AsciiDoc files in the docs directory.

This change only affects the docs directory to keep the amount of changes something I could reasonably review. The only difference when processing the docs before and after this change is the last updated timestamp, so this change has no functional impact.

The command I used to perform this was:
```sh
cd docs; for i in $(git grep -lP '\s+$'); do [[ -z ${i%%*.pdf} ]] && continue;  ruby -pi -e '$_.gsub!(/\s+$/, "\n")' $i; done
```