Remove undocumented symbols at the end of the parsing process

This ensures that we do not mark symbols as undocumented before
all potential dependencies are parsed.
Only add symbols to the unexported array once

This basically fixes a typo that caused unexported symbols to
be added to the unexported array multiple times.
Also consider linked symbols as documented
Make ol.render.canvas.Immediate available in the docs
Make ol.View2D available when exploring olx.MapOptions
Do not query the array twice
Merge pull request #1904 from ahocevar/doc-improvements

API doc improvements