Make sure 'replace' acts on correct buffer

The 'correct' buffer being the one the cursor was in when Unite was
called with the 'spell_suggest' source. This allows the user to switch
buffers with the spell suggestion window open (for reference, maybe) and
still get correct substitutions.
Add to in-line documentation (aka comments)
Clearer variable naming

For consistency with Vim's terminology in `getpos()` etc.