Update nfs.md with information on AWS EFS and Burst Credit usage and how to...

Update nfs.md with information on AWS EFS and Burst Credit usage and how to increase Burst Credit limits and also limit the need for AWS to access EFS by using FS Cache.

doc/administration/high_availability/nfs.md

@@ -42,6 +42,10 @@ GitLab does not recommend using EFS with GitLab.
   are allocated. For smaller volumes, users may experience decent performance
   for a period of time due to 'Burst Credits'. Over a period of weeks to months
   credits may run out and performance will bottom out.
+- To keep "Burst Credits" available, it may be necessary to provision more space
+  with 'dummy data'.  However, this may get expensive.
+- Another option to maintain "Burst Credits" is to use FS Cache on the server so
+  that AWS doesn't always have to go into EFS to access files.
 - For larger volumes, allocated IOPS may not be the problem. Workloads where
   many small files are written in a serialized manner are not well-suited for EFS.
   EBS with an NFS server on top will perform much better.