Fix harmless compiler warnings from clang scan-build.
Fix documentation typos.  No changes to code.
Add comments identifing where the skip-scan option is decided in the
query planner, to aid in tuning that decision.  No changes to code.
Fix an requirement mark in a test script so that it matches the
typo-corrected requirement.  No changes to code.
Avoid seeking on the main data table during the first loop of an UPDATE
if an index is sufficient to check the WHERE clause.