Rollup merge of #82645 - rkjnsn:patch-3, r=Mark-Simulacrum

Clarify that SyncOnceCell::set blocks.

Reading the discussion of this feature, I gained the mistaken impression that neither `set` nor `get` blocked, and thus calling `get` immediately after `set` was not guaranteed to succeed. It turns out that `set` *does* block, guaranteeing that the cell contains a value once `set` returns. This change updates the documentation to state that explicitly.

Happy to adjust the wording as desired.