Add portal help texts, rewrite help of other items

mods/ITEMS/mcl_fire/flint_and_steel.lua

@@ -1,8 +1,8 @@
 -- Flint and Steel
 minetest.register_tool("mcl_fire:flint_and_steel", {
 	description = "Flint and Steel",
-	_doc_items_longdesc = "Flint and steel is a tool to start fires, ignite blocks and open portals.",
+	_doc_items_longdesc = "Flint and steel is a tool to start fires and ignite blocks.",
-	_doc_items_usagehelp = "Rightclick the surface of a block to attempt to light a fire in front of it. On netherrack it will start an eternal fire. Using it on TNT will ignite it. To open a Nether portal, place an upright frame of obsidian with a length of 4 and a height of 5 blocks, leaving only air in the center. After placing this frame, use the flint and steel on inside of the frame.",
+	_doc_items_usagehelp = "Rightclick the surface of a block to attempt to light a fire in front of it or ignite the block. A few blocks have an unique reaction when ignited.",
 	inventory_image = "mcl_fire_flint_and_steel.png",
 	liquids_pointable = false,
 	stack_max = 1,

mods/ITEMS/mcl_portals/portal_end.lua

@@ -396,15 +396,19 @@ minetest.register_abm({
 
 --[[ ITEM OVERRIDES ]]
 
+local portal_open_help = "To open an End portal, place an upright frame of quartz blocks with a length of 4 blocks and a height of 5 blocks, leaving only air in the center. After placing this frame, use an eye of ender on the frame. The eye of ender is destroyed in the process."
+
 -- Frame material
 minetest.override_item(portal_frame, {
+	_doc_items_longdesc = "A block of quartz can be used to create End portals.",
+	_doc_items_usagehelp = portal_open_help,
 	on_destruct = destroy_portal,
 })
 
 -- Portal opener
 minetest.override_item("mcl_end:ender_eye", {
-	_doc_items_longdesc = "An eye of ander can be used to open a portal to the End.",
+	_doc_items_longdesc = "An eye of ender can be used to open End portals.",
-	_doc_items_usagehelp = "To open an End portal, place an upright frame of quartz blocks with a length of 4 and a height of 5 blocks, leaving only air in the center. After placing this frame, use the eye of ender on the frame.",
+	_doc_items_usagehelp = portal_open_help,
 	on_place = function(itemstack, user, pointed_thing)
 		-- Use pointed node's on_rightclick function first, if present
 		local node = minetest.get_node(pointed_thing.under)

mods/ITEMS/mcl_portals/portal_nether.lua

@@ -429,8 +429,13 @@ minetest.register_abm({
 
 --[[ ITEM OVERRIDES ]]
 
--- Frame material
+local longdesc = minetest.registered_nodes["mcl_core:obsidian"]._doc_items_longdesc
+longdesc = longdesc .. "\n" .. "Obsidian is also used as the frame of Nether portals."
+local usagehelp = "To open a Nether portal, place an upright frame of obsidian with a width of 4 blocks and a height of 5 blocks, leaving only air in the center. After placing this frame, ignite the obsidian with an appropriate tool, such as flint of steel."
+
 minetest.override_item("mcl_core:obsidian", {
+	_doc_items_longdesc = longdesc,
+	_doc_items_usagehelp = usagehelp,
 	on_destruct = destroy_portal,
 	_on_ignite = function(user, pointed_thing)
 		local pos = pointed_thing.under