doc: use better wording to explain type-aware completion

doc: use better wording to explain type-aware completion

Some editors can filter the autocompletion suggestions based on
whether the code will compile once autocompleted. Explain this
feature with better wording.

Change-Id: I29e4b0396878f18c79208915402c0a209a813b04
Reviewed-on: https://go-review.googlesource.com/53355
Reviewed-by: Florin Patan <florinpatan@gmail.com>
Reviewed-by: Andrew Bonventre <andybons@golang.org>