Add package docs to seqfile and ramfs

These are the only packages missing docs:
https://godoc.org/gvisor.dev/gvisor

PiperOrigin-RevId: 254261022

pkg/sentry/fs/proc/seqfile/seqfile.go

@@ -12,6 +12,7 @@
 // See the License for the specific language governing permissions and
 // limitations under the License.
 
+// Package seqfile provides dynamic ordered files.
 package seqfile
 
 import (

pkg/sentry/fs/ramfs/dir.go

@@ -12,6 +12,7 @@
 // See the License for the specific language governing permissions and
 // limitations under the License.
 
+// Package ramfs provides the fundamentals for a simple in-memory filesystem.
 package ramfs
 
 import (