Add a bit of detail in selection.room() documentation

It wasn't clear to me how selection.room() handles room edges and
unusual terrain in the room, so I looked at the code and wrote down how
it behaves for posterity.

I don't believe the one room currently capable of getting a random fill
while already containing some odd terrain, "Blocked center", actually
has unusual terrain in the room fill. This is because filler_region
creates an irregular region (i.e. a room containing the ROOM points in a
square ring around the blocked center). The points in the middle don't
share the same roomno, so they won't be returned in the selection
created by selection.room(). But there's no reason a room couldn't be
added in the future which does specify some nonstandard terrain and then
a themeroom fill.

doc/lua.adoc

@@ -1319,6 +1319,8 @@ Example:
 === room
 
 Create a selection of locations inside the (current) room.
+Does not include the edges of the room, such as its walls.
+Does not do any check on terrain type, so if there are non-ROOM locations inside the room, they remain part of the selection.
 
 Example: