Incorrect treatment of the default symbol

[vspinu]

Currently devdocs-lookup uses symbol at point as a default entry. This means that

1. When the current symbol is not part of thecandidates it is still placed as a default in the completion list. Selecting it results in error.
2. The list is not narrowed down to the current input

I think what most users expect is to have the list narrowed to the symbol-at-point and not to include an invalid match as a default:

[astoff]

Let's discuss in the related pull request you just opened.