spec: describe processing of function arguments for type inference more precisely

The outcome of type inference depends critically on when function
argument type inference stops processing arguments. Describe this
and explain an example with some detail.

Also: In the section on the built-in function delete, refer to the
value rather than the type of the second argument, as it may be an
untyped constant.

Change-Id: Ice7fbb33f985afe082380b8d37eaf763238a3818
Reviewed-on: https://go-review.googlesource.com/c/go/+/385034


Trust: Robert Griesemer <gri@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>