Fix some comments

Add missing word in slay.txt.  Use the Javadoc/doxygen lead-in for the comments describing the square_apparent_*() calls.

lib/gamedata/slay.txt

@@ -1,6 +1,6 @@
 # File: slay.txt
 
-# This file the slays that can appear on objects.
+# This file encodes the slays that can appear on objects.
 
 # The name and verbs can generally be edited without bad effects though slays
 # for the same type of creature should all use the same name.  Changes to the

src/cave-square.c

@@ -1808,7 +1808,7 @@ int square_digging(struct chunk *c, struct loc grid) {
 	return 0;
 }
 
-/*
+/**
  * Return the name for the terrain in a grid.  Accounts for the fact that
  * some terrain mimics another terrain.
  *
@@ -1823,7 +1823,7 @@ const char *square_apparent_name(struct chunk *c, struct loc grid) {
 	return f_info[f].name;
 }
 
-/*
+/**
  * Return the prefix, appropriate for describing looking at the grid in
  * question, for the name returned by square_name().
  *
@@ -1841,7 +1841,7 @@ const char *square_apparent_look_prefix(struct chunk *c, struct loc grid) {
 		(is_a_vowel(f_info[f].name[0]) ? "an " : "a ");
 }
 
-/*
+/**
  * Return a preposition, appropriate for describing the grid the viewer is on,
  * for the name returned by square_name().  May return an empty string when
  * the name doesn't require a preposition.