Those fifos are accessible during %sh{...} blocks, the command fifo
executes commands written to it once the write end side is closed
(multiple open/write/close sequences are supported), the response
fifo is a simple helper fifo available to write response back to
the shell process
An example use of this feature is to request some list options
content from without being limited by the environment size:
```
%sh{
echo "echo -to-file $kak_response_fifo -quoting shell -- %opt{some_list}" > $kak_command_fifo
eval "set -- $(cat $kak_response_fifo)"
}
```
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.
Also, moved the "Markup strings" section to faces.asciidoc, since it wasn't
anything to do with expansions, and updated various hyperlinks to point at the
new location.
Now that we have a nice standard way to express lists of strings,
registers can be fully exposed. An new $kak_main_reg_... env var
was added to provide the previous behaviour which is relied on by
doc.kak.
