Add `rethrowSTM` and improve STM-related documentation

Adding rethrowSTM resolves #26758 (closed) and implements CLC proposal #436.

The implementation of rethrowSTM is completely analogous to the one of rethrowIO.

The following is established for the documentation of throwSTM and catchSTM:

• Both operations are directly described as analogs of their IO counterparts.

• There is no reference to throw in the documentation of throwSTM, because, although such a reference is great in the documentation of throwIO, it is somewhat out of place in the documentation of throwSTM.

• Instead of repeating part of throwIO’s documentation, the documentation of throwSTM just recommends using throwSTM instead of throw and references the corresponding arguments in the documentation of throwIO.