Add help option to bootchart man page

Bootchart has a help option. For the sake of consistency, this patch
adds it to the man page.

Also, the TODO is updated. Bootcharts were added to the journal in
commit c4d58b0.

TODO

@@ -592,7 +592,6 @@ Features:
    - plot per-process IO utilization
    - group processes based on service association (cgroups)
    - document initcall_debug
-   - put bootcharts in the journal
    - kernel cmdline "bootchart" option for simplicity?
 
 External:

man/systemd-bootchart.xml

@@ -145,6 +145,12 @@
 
                 <variablelist>
 
+                        <varlistentry>
+                               <term><option>-h</option></term>
+                               <term><option>--help</option></term>
+                               <listitem><para>Prints a short help text and exits.</para></listitem>
+                        </varlistentry>
+
                         <varlistentry>
                                 <term><option>-n</option></term>
                                 <term><option>--sample <replaceable>N</replaceable></option></term>