speakup: Document read_all_doc shortcut

This was implemented a long time ago, but never actually added to the
documentation.

Signed-off-by: Samuel Thibault <samuel.thibault@ens-lyon.org>
Link: https://lore.kernel.org/r/20201108181824.bso5exam72b4p4tk@function
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
diff --git a/Documentation/admin-guide/spkguide.txt b/Documentation/admin-guide/spkguide.txt
index 3782f6a..5ff6a0f 100644
--- a/Documentation/admin-guide/spkguide.txt
+++ b/Documentation/admin-guide/spkguide.txt
@@ -344,6 +344,7 @@
 spk key_8 = speakup_paste
 shift spk key_m = say_first_char
  ctrl spk key_semicolon = say_last_char
+spk key_r = read_all_doc
 
 5.  The Speakup Sys System