update descriptions for deep water and lava

crawl-ref/source/dat/descript/features.txt

@@ -150,14 +150,16 @@ The floor.
 %%%%
 Some deep water
 
-A pool of deep water. Only very large creatures can wade through this, and only
-those well-adapted to water can swim through it; for the rest, the only way
-through is to fly over.
+A pool of deep water. Very large creatures can wade through this, and
+those well-adapted to water can easily swim through it. Less-adapted
+species can swim through it with difficulty, but will be unable to 
+attack or cast while doing so. Most other dungeon monsters will just drown.
 %%%%
 Some lava
 
 An intensely hot pool of molten rock, emitting clouds of sulphurous smoke.
-Nothing but the mythical salamander can cross this without flight.
+Nothing but the mythical salamander can step through this without extreme 
+pain and difficulty, but it is possible to safely fly across.
 %%%%
 Some shallow water