expansions.asciidoc: Add an example of file expansion.

[Screwtapello]

Properly speaking, all expansions are of the form "%abc{}" where "abc" is the expansion type. All expansion types are listed in the documentation, as they should be. However, when searching through the documentation, a keyword like "file" is likely to hit a lot of false-positives, so a smart user is likely to search for the more-unique "%file" instead. Unfortunately, unlike every other expansion-type, file-expansions did not include an example, so a search for "%file" resulted in no hits, potentially leading people to believe it was undocumented.

Now there's an example, so people searching for "%file" will find what they're looking for.

[Screwtapello]

Admission: Earlier today I wanted to link somebody to the %file{} docs, so I searched for %file. When I didn't find it, I decided to make a PR documenting it, only to find it had actually been documented all along. So this is as much for the benefit of my laziness as anybody else. :)

[alexherbo2]

How to handle arbitrary filenames?

[Screwtapello]

In some circumstances (such as piping to kak -p) you can use NUL as the argument delimiter. Since NUL cannot legally appear in a filename, it's safe to use it to delimit filenames. However, bash doesn't like NUL bytes appearing in command-substitutions, and I can't figure out how to type NUL directly in bash, or in Kakoune, so it's not a universally-applicable solution.

[alexherbo2]

@Screwtapello It could be nice to add an example with a filename from an option.

[Screwtapello]

It would, but off the top of my head I'm not even sure how to go about it. I suspect it would be too complex to make a good example.