Add Doxygen documentation for LS regression and parametric exercise

[Aida118]

Two header files in ql/methods/montecarlo/ had TODO comments asking for
documentation. This fills them in — nothing else changed.

[boring-cyborg]

Thanks for opening this pull request! It might take a while before we look at it, so don't worry if there seems to be no feedback. We'll get to it.

[CLAassistant]

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[lballabio]

Thanks! May you check the comment above from CLAassistant?