docs(lsp): remove private lsp.diagnostic functions from docs (#15541)

Both `apply_to_diagnostic_items` and `show_diagnostics` are local
functions and cannot be called by users.

runtime/lua/vim/lsp/diagnostic.lua

@@ -1214,9 +1214,8 @@ function M.redraw(bufnr, client_id)
   )
 end
 
--- }}}
--- Diagnostic User Functions {{{
 
+---@private
 --- Open a floating window with the provided diagnostics
 ---
 --- The floating window can be customized with the following highlight groups:
@@ -1265,8 +1264,11 @@ local function show_diagnostics(opts, diagnostics)
   return popup_bufnr, winnr
 end
 
---- Open a floating window with the diagnostics from {position}
 
+-- }}}
+-- Diagnostic User Functions {{{
+
+--- Open a floating window with the diagnostics from {position}
 ---@param opts table|nil Configuration keys
 ---         - severity: (DiagnosticSeverity, default nil)
 ---             - Only return diagnostics with this severity. Overrides severity_limit
@@ -1334,6 +1336,7 @@ function M.reset(client_id, buffer_client_map)
   end)
 end
 
+---@private
 --- Gets diagnostics, converts them to quickfix/location list items, and applies the item_handler callback to the items.
 ---@param item_handler function Callback to apply to the diagnostic items
 ---@param command string|nil Command to execute after applying the item_handler