Clarify ExternalStorage documentation
Clarify ExternalStorage documentation

anirajk/RAMCloud,anirajk/RAMCloud,SMatsushi/RAMCloud,anirajk/RAMCloud,alexandermerritt/ramcloud,jblomer/ramcloud,alexandermerritt/ramcloud,matrix207/RAMCloud,jblomer/ramcloud,QingkaiLu/RAMCloud,utah-scs/RAMCloud,QingkaiLu/RAMCloud,utah-scs/RAMCloud,rstutsman/RAMCloud,rstutsman/RAMCloud,utah-scs/RAMCloud,SMatsushi/RAMCloud,matrix207/RAMCloud,jblomer/ramcloud,rstutsman/RAMCloud,jblomer/ramcloud,matrix207/RAMCloud,anirajk/RAMCloud,SMatsushi/RAMCloud,matrix207/RAMCloud,utah-scs/RAMCloud,SMatsushi/RAMCloud,rstutsman/RAMCloud,rstutsman/RAMCloud,alexandermerritt/ramcloud,alexandermerritt/ramcloud,QingkaiLu/RAMCloud,jblomer/ramcloud,alexandermerritt/ramcloud,QingkaiLu/RAMCloud,QingkaiLu/RAMCloud,matrix207/RAMCloud,alexandermerritt/ramcloud,QingkaiLu/RAMCloud,SMatsushi/RAMCloud,anirajk/RAMCloud,matrix207/RAMCloud,jblomer/ramcloud,anirajk/RAMCloud,utah-scs/RAMCloud,SMatsushi/RAMCloud,rstutsman/RAMCloud,utah-scs/RAMCloud