Guidebook update: map display of mons and objs

Mostly add a paragraph clarifying the classification of WONTFIX for
a reported bug ('I' unseen monster clobbers remembered object at the
location).  It felt like it needed a parallel paragraph for monsters.

doc/Guidebook.tex

@@ -1915,6 +1915,23 @@ however, they are likely to be cursed.  Beware of whatever killed the
 former player; it is probably still lurking around, gloating over its
 last victory.
 
+%.hn 2
+\subsection*{Persistence of Monsters}
+
+%.pg
+Monsters (a generic reference which includes humans and pets) are only
+shown while they can be seen or otherwise sensed.
+Moving to a location where you can't see or sense a monster any more
+will result in it disappearing from your map.
+
+%.pg
+However, if you encounter a monster which you can't see or sense--
+perhaps it is invisible and has just tapped you on the noggin--
+a special ``remembered, unseen monster'' marker will be displayed at
+the location where you think it is.  That will persist until you have
+proven that there is no monster there, even if the unseen monster
+moves to another location or dies.
+
 %.hn 1
 \section{Objects}
 
@@ -2545,6 +2562,32 @@ There are a number
 of monsters in the dungeon that may be influenced by the amount of gold
 you are carrying (shopkeepers aside).
 
+%.hn 2
+\subsection*{Persistence of Objects}
+
+%.pg
+Normally, if you have seen an object at a particular map location and
+move to another location which can't directly see that object any
+more, if will continue to be displayed on your map.
+That remains the case even if it is not actually there any more--
+perhaps a monster has picked it up or it has rotted away--
+until you can see or feel that location again.
+One notable exception is that if the object gets covered by the
+``remembered, unseen monster'' marker and that marker is later removed
+after you've verified that no monster is there, you will forget that
+there was any object there regardless of whether the unseen monster
+actually took the object.
+If the object is still there, then once you see or feel that location
+again you will re-discover the object and resume remembering it.
+
+%.pg
+The situation is the same for a pile of objects, except that only the
+top item of the pile is displayed.
+The
+{\it hilite\verb+_+pile\/}
+option can be enabled in order to show an item differently when is
+the top one of a pile.
+
 %.hn 1
 \section{Conduct}