github PR #1052 - levltyp[] re-synchronization

Pull request from entrez:  the legend for wizard mode #terrain wasn't
updated to include terrain type "lava wall", so the entries for it
and everything that followed were inaccurate.

I've expanded the comment about level type codes in rm.h.

Closes #1052

include/rm.h

@@ -31,10 +31,27 @@
  * +-           -+-             -+              |
  */
 
-/* Level location types.  [Some debugging code in src/display.c
-   defines array type_names[] which contains an entry for each of
-   these, so needs to be kept in sync if any new types are added
-   or existing ones renumbered.] */
+/*
+ * Level location types, values for 'level.locations[x][y].typ'.
+ *
+ * These are different from display symbols and while there is
+ * similarity between the cmap subset of those, there isn't any
+ * one-to-one correspondence between the two encodings.  For instance,
+ * the DOOR type has multiple symbols (closed|open|gone); so does the
+ * STAIRS type (down|up).  Conversely, DRAWBRIDGE_UP represents the
+ * location of the projected span if the drawbridge were down but
+ * there is no symbol for that; it is displayed as water, ice, lava, or
+ * floor depending on what is at that spot when the bridge is 'closed'.
+ *
+ * [Some debugging code in src/display.c defines array type_names[]
+ * which contains an entry for each of these, so needs to be kept in
+ * sync if any new types are added or existing ones renumbered.  The
+ * #terrain command also has a menu choice to display each map spot by
+ * a letter derived from these numeric values and another choice to
+ * display a legend showing the letter-to-type correspondence.  If any
+ * types are added, removed, or reordered, that needs to be updated to
+ * keep in synch.]
+ */
 enum levl_typ_types {
     STONE     =  0,
     VWALL     =  1,