ALL comment fixes

This is every typo or comment fix I could find in the issues backlog (plus a few more I manually fixed by browsing source and tests).

This single PR will automatically close the following issues when merged: #1243, #1242, #1507, #1351, #1373, #1386, #1441, #1513, #1537, #1544, #1673, #1682, #1713, #1806, #1871, #1872, #1877, #1897

These commits *only* modify spelling and trailing whitespace.  There are no code functionality changes introduced.