doc: convert the remaining legacy syntax

The block conversion changed the bodies of the reference sections but
left every "**Syntax:**" header spelling the line-based format, so each
page opened by teaching the format it then stopped using.  Six files
were missed entirely: runparts, files, capabilities, requirements,
runlevels, and switchroot.

runparts had no block spelling written down anywhere, though the parser
has read `runparts`, `runparts-progress`, and `runparts-sysv` all along.

tty gains a table per variant.  Its three syntax lines carried nine
positional fields between them, which no longer describes anything the
parser accepts.

Fixes #148

Signed-off-by: Joachim Wiberg <troglobit@gmail.com>

doc/requirements.md

@@ -15,7 +15,11 @@ done slightly differently and on systems with udev you might want to add
 the following one-shot task early in your `/etc/finit.conf`:
 
 ```conf
-run [S] udevadm settle --timeout=120 -- Waiting for udev
+run udevadm {
+    description = "Waiting for udev"
+    runlevel    = "S"
+    command     = "udevadm settle --timeout=120"
+}
 ```
 
 Finit has a built-in Getty for TTYs, but requires a working `/bin/login`