builtin: document when len and cap are constant

The rules are subtle, but under some circumstances the result
can be constant. Mention this and refer to the appropriate
section of the specification.

Fixes #27588.

Change-Id: I4beaad036db87501378fb2ef48d216742d096933
Reviewed-on: https://go-review.googlesource.com/135519


Reviewed-by: Robert Griesemer <gri@golang.org>