man: document how to get the boot menu with zero time-out

Fixes: #15125

diff --git a/man/systemd-boot.xml b/man/systemd-boot.xml
index 8c64aa21e4c24e576f7ab6aa7354e40de3ba6e1a..dacd49cd7bcf1fe95c6122202d8580cd206105ba 100644 (file)
--- a/man/systemd-boot.xml
+++ b/man/systemd-boot.xml
@@ -176,8 +176,8 @@
       </varlistentry>
     </variablelist>
 
-    <para>The following keys may be used during bootup or in the boot menu to
-    directly boot a specific entry:</para>
+    <para>The following keys may be pressed during bootup or in the boot menu to directly boot a specific
+    entry:</para>
 
     <variablelist>
       <varlistentry>
@@ -214,6 +214,17 @@
       </varlistentry>
     </variablelist>
 
+    <para>The boot menu is shown when a non-zero menu timeout has been configured. If the menu timeout has
+    been set to zero, it is sufficient to press any key — before the boot loader initializes — to bring up
+    the boot menu, except for the keys listed immediately above as they directly boot into the selected boot
+    menu item. Note that depending on the firmware implementation the time window where key presses are
+    accepted before the boot loader initializes might be short. If the window is missed, reboot and try
+    again, possibly pressing a suitable key (e.g. the space bar) continuously; on most systems it should be
+    possible to hit the time window after a few attempts. To avoid this problem, consider setting a non-zero
+    timeout, thus showing the boot menu unconditionally. Some desktop environments might offer an option to
+    directly boot into the boot menu, to avoid the problem altogether. Alternatively, use the command line
+    <command>systemctl reboot --boot-loader-menu=0</command> from the shell.</para>
+
     <para>In the editor, most keys simply insert themselves, but the following keys
     may be used to perform additional actions:</para>