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/plugins.md

@@ -71,7 +71,7 @@ For your convenience a set of *optional* plugins are available:
   comment character, `#`, or `;`, is skipped.
   
   Modules are by default loaded in runlevel `S` using the `task` stanza.
-  Each module gets a unique `name:modprobe.foo`, and optional`:ID`.  The
+  Each module is named `modprobe.foo`, with an optional `:ID`.  The
   runlevel can be changed per file using:
 
         set runlevel 2345