docs(copilot): add instructions to enable lsp inline completion (#4111)

Author: ateoi

lsp/copilot.lua

@@ -13,6 +13,33 @@
 ---
 --- Please see [terms of use for GitHub Copilot](https://docs.github.com/en/site-policy/github-terms/github-terms-for-additional-products-and-features#github-copilot)
 ---
+--- You need to enable `:help lsp-inline-completion` to receive suggestions. For example, you can enable it in the LspAttach event:
+---
+--- ```lua
+--- vim.api.nvim_create_autocmd('LspAttach', {
+---   callback = function(args)
+---     local bufnr = args.buf
+---     local client = assert(vim.lsp.get_client_by_id(args.data.client_id))
+---
+---     if client:supports_method(vim.lsp.protocol.Methods.textDocument_inlineCompletion, bufnr) then
+---       vim.lsp.inline_completion.enable(true, { bufnr = bufnr })
+---
+---       vim.keymap.set(
+---         'i',
+---         '<C-F>',
+---         vim.lsp.inline_completion.get,
+---         { desc = 'LSP: accept inline completion', buffer = bufnr }
+---       )
+---       vim.keymap.set(
+---         'i',
+---         '<C-G>',
+---         vim.lsp.inline_completion.select,
+---         { desc = 'LSP: switch inline completion', buffer = bufnr }
+---       )
+---     end
+---   end
+--- })
+--- ```
 
 ---@param bufnr integer,
 ---@param client vim.lsp.Client