doc: remove "known bug" about global variables in debug_info.

This hasn't been true at least since 1.4. Until golang.org/cl/137235
they were lumped together into a random compile unit, now they are
assigned to the correct one.

Change-Id: Ib66539bd67af3e9daeecac8bf5f32c10e62e11b1
Reviewed-on: https://go-review.googlesource.com/138415


Reviewed-by: Than McIntosh <thanm@google.com>
Reviewed-by: David Chase <drchase@google.com>