This paragraph has been added, as the notion was missing from the
documentation.

If a value is passed to Encode and the type is not a struct (or pointer to struct,
etc.), for simplicity of processing it is represented as a struct of one field.
The only visible effect of this is to encode a zero byte after the value, just as
after the last field of an encoded struct, so that the decode algorithm knows when
the top-level value is complete.

Fixes #16978

Change-Id: I5f008e792d1b6fe80d2e026a7ff716608889db32
Reviewed-on: https://go-review.googlesource.com/38414Reviewed-by: Ian Lance Taylor <iant@golang.org>