More Guidebook menu_* settings

Previous update added one change to Guidebook.tex which was omitted
from Guidebook.mn, so catch up.

Don't try to list the interfaces or ports which support each menu
command key.  Just have a generic "not every command is implemented
by every interface" instead.

Flesh out the one sentence menu_search description.

Update an example which shows changing menu_first_page to '{' now
that '{' has a default use for something else (menu_shift_left).
Since 'menu_first_page:^' is the opposite of 'menu_last_page:|' and
'\' shares the same key as '|' with opposite shift state, switch
the example to 'menu_first_page:\'.

doc/Guidebook.tex

@@ -4152,7 +4152,6 @@ Implemented by the Amiga, Gem and tty ports.
 Default `\verb+<+'.
 \item[\ib{menu\verb+_+search}]
 Key to search for a menu item.
-Implemented by the Amiga, Gem, X11 and tty ports.
 Default `:'.
 \item[\ib{menu\verb+_+select\verb+_+all}]
 Key to select all items in a menu.
@@ -5165,7 +5164,7 @@ For example:
 
 \begin{verbatim}
     BIND=^X:getpos.autodescribe
-    BIND={:menu_first_page
+    BIND=\:menu_first_page
     BIND=v:loot
 \end{verbatim}
 
@@ -5181,6 +5180,7 @@ the ``{\tt <esc>}'', ``{\tt <enter>}'', and ``{\tt <space>}'' keys.
 The menu control or accelerator keys can also be rebound via OPTIONS lines
 in the configuration file.
 You cannot bind object symbols or selection letters into menu accelerators.
+Some interfaces only support some of the menu accelerators.
 
 %.lp "Special command keys"
 \item[\tb{Special command keys}]