Clarifying a comment.
Better completion in the middle of a word

For instance (`|` represents the cursor):
  1. Buffer state: `foo.|bar`
  2. A completion candidate of `zoobar` is shown and the user selects it.
  3. Buffer state: `foo.zoobar|bar` instead of `foo.zoo|bar` which is what the
  user wanted.

This commit resolves that issue.

It could be argued that the user actually wants the final buffer state to be
`foo.zoobar|` (the cursor at the end), but that would be much more difficult
to implement and is probably not worth doing.

Fixes #374.