docs(lua): fix, clarify Lua require() docs

Corrects lua.txt help file to say that require() searches runtimepath
and loads the first module found, not the last.

Also adds additional clarification on require() and module search order.

Closes #15480

runtime/doc/lua.txt

@@ -21,19 +21,35 @@ Nvim includes a "standard library" |lua-stdlib| for Lua.  It complements the
 be used from Lua code. A good overview of using Lua in neovim is given by
 https://github.com/nanotee/nvim-lua-guide.
 
-Module conflicts are resolved by "last wins".  For example if both of these
+The |:source| and |:runtime| commands can run Lua scripts as well as Vim
-are on 'runtimepath':
+scripts.  Lua modules can be loaded with `require('name')`, which
-    runtime/lua/foo.lua
+conventionally returns a table but can return any value.
-    ~/.config/nvim/lua/foo.lua
+
-then `require('foo')` loads "~/.config/nvim/lua/foo.lua", and
+See |lua-require| for details on how Nvim finds and loads Lua modules.
-"runtime/lua/foo.lua" is not used.  See |lua-require| to understand how Nvim
+See |lua-require-example| for an example of how to write and use a module.
-finds and loads Lua modules.  The conventions are similar to those of
-Vimscript |plugin|s, with some extra features.  See |lua-require-example| for
-a walkthrough.
 
 ==============================================================================
 IMPORTING LUA MODULES                                   *lua-require*
 
+Modules are searched for under the directories specified in 'runtimepath', in
+the order they appear.  For a module `foo`, each directory is searched for
+`lua/foo.lua`, then the entire list is searched again for `lua/foo/init.lua`,
+then paths specified by `package.path` and `package.cpath`.  The first script
+found is run and `require()` returns the value returned by the script if any,
+else `true`.
+
+The return value is cached after the first call to `require()` for each
+module, with subsequent calls returning the cached value without searching for
+or executing any script.  For further details on `require()`, see the Lua
+documentation at https://www.lua.org/manual/5.1/manual.html#pdf-require.
+
+For example, if 'runtimepath' is "foo,bar", `require('mod')` searches these
+paths in order and the first script found is used:
+    foo/lua/mod.lua
+    bar/lua/mod.lua
+    foo/lua/mod/init.lua
+    bar/lua/mod/init.lua
+
                                                         *lua-package-path*
 Nvim automatically adjusts `package.path` and `package.cpath` according to
 effective 'runtimepath' value.  Adjustment happens whenever 'runtimepath' is