remove comment about API being undecided

Enough time has passed since the API was created, and so far, I have
not had desires to change it, nor has anyone else suggested any API
improvements. It's likely going to stay as is, so remove the comment.
If there are good change ideas in the future, it can still be
considered. But the comment was meant for the very early days of the
package's existence, and has outlived its usefulness.
dmitshur committed 4 years ago commit b1254a4463635f9a4eddb19e8fa1e379f1383e20
@@ -9,13 +9,10 @@
// The second priority is performance. The current version uses bufio.Reader and
// ReadBytes. Performance can be optimized further by using lower level I/O
// primitives and allocating less. That can be explored later. A lot of the time
// is spent on reading the entire file without being able to stop early,
// since the specification allows the comment to appear anywhere in the file.
// The exact API is undecided and can change. The current API style is somewhat
// based on go/parser, but that may not be the best approach.
package generated

import (