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@functionSigned-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Documentation/admin-guide/spkguide.txt

@@ -344,6 +344,7 @@ spk key_slash = say_attributes
 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