Add explanations of textures & player api usage, non-function sounds, and some minor touch-up of docs (#55)

doc/functions.md

@@ -1,13 +1,16 @@
 # Functions API
 
 ## `can_interact_with_node(player, pos)`
+
 returns `bool`
 
 checks for the ability to interact with a node via:
+
 * if a player
 * owner metadata key
-* protection_bypass
+* `protection_bypass`
 
 supports
+
 * minetest game default if present
-* else polyfill
+* else polyfill