mention `api-global`

Fix #76

README.md

@@ -400,7 +400,7 @@ Some notable functions and modules include:
 - `vim.lsp`: module that controls the built-in LSP client
 - `vim.treesitter`: module that exposes the functionality of the tree-sitter library
 
-This list is by no means comprehensive. If you wish to know more about what's made available by the `vim` variable, [`:help lua-stdlib`](https://neovim.io/doc/user/lua.html#lua-stdlib) and [`:help lua-vim`](https://neovim.io/doc/user/lua.html#lua-vim) are the way to go. Alternatively, you can do `:lua print(vim.inspect(vim))` to get a list of every module.
+This list is by no means comprehensive. If you wish to know more about what's made available by the `vim` variable, [`:help lua-stdlib`](https://neovim.io/doc/user/lua.html#lua-stdlib) and [`:help lua-vim`](https://neovim.io/doc/user/lua.html#lua-vim) are the way to go. Alternatively, you can do `:lua print(vim.inspect(vim))` to get a list of every module. API functions are documented under [`:help api-global`](https://neovim.io/doc/user/api.html#api-global).
 
 #### Tips
 

doc/nvim-lua-guide.txt

@@ -492,7 +492,7 @@ tree-sitter library
 This list is by no means comprehensive. If you wish to know more about what's
 made available by the `vim` variable, |lua-stdlib| and |lua-vim| are the way
 to go. Alternatively, you can do `:lua print(vim.inspect(vim))` to get a list
-of every module.
+of every module. API functions are documented under |api-global|.
 
 Tips~