This commit moves the documentation about how Go values are mapped to
JavaScript values to the functions that apply the mapping, instead of
mentioning them in the documentation of the types being mapped. This
should be easier to read.

Change-Id: I2465eb4a45f71b3b61624349e908a195010a09f1
Reviewed-on: https://go-review.googlesource.com/126856Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>