doc: convert the documentation to the block format

The syntax overview no longer describes a line-based format, since that
is not what the rest of the documentation shows.  It now covers the
grammar, the two naming conventions, the nine aliases, and the leading
'-' on a path, and it says plainly that both formats are still read and
told apart per file by content.  Without that, a reader with an
existing configuration is left wondering what happened to it.

service-opts.md was a list of modifiers to place between a directive
and its command, so it needed rewriting rather than translating: there
are no positions left to describe.  It is now grouped by what the
settings do.

conditions.md needed correcting.  It presented '!' as a condition
prefix alongside '~'.  It is neither a condition nor a negation, it is
a flag on the block that means one thing on a service and another on a
run or task, so it is spelled reload-signal and required here, and the
page maps the old form to both.

Two things the pages claimed are not true.  The kill delay range is
1-300, not 1-60, and stop and reload scripts are no longer run without
a timeout.

ChangeLog.md keeps its line-based examples.  Those sit in historical
release entries, and rewriting them in a syntax that did not exist at
the time would misdate the format.

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

doc/features.md

@@ -212,13 +212,17 @@ A fourth group also exists, the `root` group.  It is also _reserved_ and
 primarily intended for RT tasks.  If you have RT tasks they need to be
 declared as such in their service stanza like this:
 
-    service [...] <...> cgroup.root /path/to/foo args -- description
+    service foo {
+        cgroup root {}
+        command = "/path/to/foo args"
+    }
 
-or
+Every block names the group it joins, so a second RT task says so too:
 
-    cgroup.root
-    service [...] <...> /path/to/foo args -- description
-    service [...] <...> /path/to/bar args -- description
+    service bar {
+        cgroup root {}
+        command = "/path/to/bar args"
+    }
 
 See the [Cgroups](config/cgroups.md) section for more information, e.g.,
 how to configure per-group limits.