Rollup merge of #118450 - marcin-serwin:master, r=workingjubilee

Use OnceCell in cell module documentation

The spanning tree example in the std cell module implementation was created before `OnceCell` was added to Rust so it uses `RefCell`. However, in this case using `OnceCell` seems more appropriate and produces simpler code. As a bonus, this also means that all three cell types are presented in the examples of std cell module.

Author: compiler-errors