Comment edits (#2785)
Comment edits (#2785)

* comments: standardize spelling 'maps-to notation'

* comment fix

* comment fix

* comments: add cross-links for pm3.2(i) and jca(d)

* various comment edits/fixes (non-systematic)

* comments: some edits in initial comment; other minor edits

* standardize spelling: 'indices' and 'well-order'
metamath/set.mm,metamath/set.mm,metamath/set.mm