Player_api: Various maintenance (#2737)

Clear 'player_sneak' and 'player_api.player_attached' table values
when player leaves.
Remove unnecessary commas and whitespace.
Fix table name in 'game_api.txt'.
Clean up documentation in 'game_api.txt'.

mods/player_api/init.lua

@@ -5,7 +5,7 @@ dofile(minetest.get_modpath("player_api") .. "/api.lua")
 -- Default player appearance
 player_api.register_model("character.b3d", {
 	animation_speed = 30,
-	textures = {"character.png", },
+	textures = {"character.png"},
 	animations = {
 		-- Standard animations.
 		stand     = {x = 0,   y = 79},