docs(lsp): document codelens.get bufnr parameter (#15540)

Alternative to https://github.com/neovim/neovim/pull/15224

runtime/doc/lsp.txt

@@ -1725,6 +1725,10 @@ display({lenses}, {bufnr}, {client_id})           *vim.lsp.codelens.display()*
 get({bufnr})                                          *vim.lsp.codelens.get()*
                 Return all lenses for the given buffer
 
+                Parameters: ~
+                    {bufnr}  number Buffer number. 0 can be used for the
+                             current buffer.
+
                 Return: ~
                     table ( `CodeLens[]` )
 

runtime/lua/vim/lsp/codelens.lua

@@ -44,9 +44,10 @@ end
 
 --- Return all lenses for the given buffer
 ---
+---@param bufnr number  Buffer number. 0 can be used for the current buffer.
 ---@return table (`CodeLens[]`)
 function M.get(bufnr)
-  local lenses_by_client = lens_cache_by_buf[bufnr]
+  local lenses_by_client = lens_cache_by_buf[bufnr or 0]
   if not lenses_by_client then return {} end
   local lenses = {}
   for _, client_lenses in pairs(lenses_by_client) do