Prior to this CL, there were two requirements about the
package name main.

1. The package that sits at the root of the import graph
   (the one where program execution begins)
   must be named main.

2. No other package in the program can be named main.

This CL only removes requirement #2, which can be done
without changing any other Go documentation.

The new wording and formatting is such that removing
requirement #1 can be done by deleting a single line,
but making that change is explicitly outside the scope
of this CL, because it would require changes to other
documentation at the same time.

R=gri, r, gri1
CC=golang-dev
https://golang.org/cl/4126053