reflect: use 'defined' rather than 'named', use 'embedded' rather than 'anonymous'

reflect: use 'defined' rather than 'named', use 'embedded' rather than 'anonymous'

On the API level this is just an update of the documentation to match
the current spec more closely.

On the implementation side, this is a rename of various unexported names.

For #22005.

Change-Id: Ie5ae32f3b10f003805240efcceab3d0fd373cd51
Reviewed-on: https://go-review.googlesource.com/112717
Run-TryBot: Robert Griesemer <gri@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>