sync: update misleading comment in map.go about entry type

As discussed in: https://github.com/golang/go/issues/45429,  about entry
type comments, it is possible for p == nil when m.dirty != nil, so
update the commemt about it.

Fixes #45429

Change-Id: I7ef96ee5b6948df9ac736481d177a59ab66d7d4d
GitHub-Last-Rev: 202c598a0ab98f4634cb56fe2486e8e82f9d991f
GitHub-Pull-Request: golang/go#45443
Reviewed-on: https://go-review.googlesource.com/c/go/+/308292


Reviewed-by: Changkun Ou <euryugasaki@gmail.com>
Reviewed-by: Bryan C. Mills <bcmills@google.com>
Run-TryBot: Bryan C. Mills <bcmills@google.com>
Trust: Robert Findley <rfindley@google.com>
TryBot-Result: Go Bot <gobot@golang.org>