aio / timers: De-document -clock

Commit 6d32717 "aio / timers: Remove alarm timers" has issues:

1. It silently ignores -clock for backward compatibility.
Incompatible change: -clock help no longer terminates the program.
Tolerable.

2. Failed to update option documentation.  In particular, -help still
advises users to try -clock help for available timers.  Drop all
documentation on -clock.

3. The 'query-alarm-clock' example in docs/writing-commands.txt no
longer works, and needs to be redone.  Can't do that right now, so I
just stick in a FIXME.

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

docs/writing-qmp-commands.txt

@@ -365,6 +365,8 @@ documentation for information about the other types.
 
 === User Defined Types ===
 
+FIXME This example needs to be redone after commit 6d32717
+
 For this example we will write the query-alarm-clock command, which returns
 information about QEMU's timer alarm. For more information about it, please
 check the "-clock" command-line option.

qemu-options.hx

@@ -2989,16 +2989,8 @@ Load the contents of @var{file} as an option ROM.
 This option is useful to load things like EtherBoot.
 ETEXI
 
-DEF("clock", HAS_ARG, QEMU_OPTION_clock, \
-    "-clock          force the use of the given methods for timer alarm.\n" \
-    "                To see what timers are available use '-clock help'\n",
-    QEMU_ARCH_ALL)
-STEXI
-@item -clock @var{method}
-@findex -clock
-Force the use of the given methods for timer alarm. To see what timers
-are available use @code{-clock help}.
-ETEXI
+HXCOMM Silently ignored for compatibility
+DEF("clock", HAS_ARG, QEMU_OPTION_clock, "", QEMU_ARCH_ALL)
 
 HXCOMM Options deprecated by -rtc
 DEF("localtime", 0, QEMU_OPTION_localtime, "", QEMU_ARCH_ALL)