Add note about huge pages using up memory. (#2733)

* Add note about huge pages using up memory.

* Update doc

* Update

doc/source/plasma-object-store.rst

@@ -19,8 +19,14 @@ huge pages as follows.
   uid=`id -u`
   sudo mount -t hugetlbfs -o uid=$uid -o gid=$gid none /mnt/hugepages
   sudo bash -c "echo $gid > /proc/sys/vm/hugetlb_shm_group"
+  # This typically corresponds to 20000 2MB pages (about 40GB), but this
+  # depends on the platform.
   sudo bash -c "echo 20000 > /proc/sys/vm/nr_hugepages"
 
+**Note:** Once you create the huge pages, they will take up memory which will
+never be freed unless you remove the huge pages. If you run into memory issues,
+that may be the issue.
+
 You need root access to create the file system, but not for running the object
 store.