storage: document mu.desc as pointer-to-immutable
storage: document mu.desc as pointer-to-immutable

Documents the issue pointed out in #5509.

arjun4084346/cockroach,petermattis/cockroach,vaibhavarora/cockroach,cockroachdb/cockroach,vivekmenezes/cockroach,bg451/cockroach,cockroachdb/cockroach,glycerine/cockroach,paperstreet/cockroach,veteranlu/cockroach,glycerine/cockroach,veteranlu/cockroach,vaibhavarora/cockroach,arjun4084346/cockroach,paperstreet/cockroach,vaibhavarora/cockroach,veteranlu/cockroach,vaibhavarora/cockroach,vivekmenezes/cockroach,vivekmenezes/cockroach,paperstreet/cockroach,vivekmenezes/cockroach,sunya123/cockroach,maxlang/cockroach,maxlang/cockroach,glycerine/cockroach,glycerine/cockroach,paperstreet/cockroach,petermattis/cockroach,sunya123/cockroach,veteranlu/cockroach,cockroachdb/cockroach,sunya123/cockroach,vaibhavarora/cockroach,arjun4084346/cockroach,petermattis/cockroach,vaibhavarora/cockroach,cockroachdb/cockroach,veteranlu/cockroach,vivekmenezes/cockroach,maxlang/cockroach,petermattis/cockroach,maxlang/cockroach,sunya123/cockroach,vivekmenezes/cockroach,veteranlu/cockroach,sunya123/cockroach,cockroachdb/cockroach,bg451/cockroach,arjun4084346/cockroach,arjun4084346/cockroach,maxlang/cockroach,sunya123/cockroach,arjun4084346/cockroach,maxlang/cockroach,glycerine/cockroach,glycerine/cockroach,cockroachdb/cockroach,bg451/cockroach,arjun4084346/cockroach,cockroachdb/cockroach,maxlang/cockroach,bg451/cockroach,petermattis/cockroach,petermattis/cockroach,vaibhavarora/cockroach,sunya123/cockroach,glycerine/cockroach,paperstreet/cockroach,paperstreet/cockroach,paperstreet/cockroach,vivekmenezes/cockroach,paperstreet/cockroach,glycerine/cockroach,bg451/cockroach,veteranlu/cockroach,vaibhavarora/cockroach,petermattis/cockroach,bg451/cockroach,maxlang/cockroach,cockroachdb/cockroach,vivekmenezes/cockroach,arjun4084346/cockroach,sunya123/cockroach,bg451/cockroach,veteranlu/cockroach,petermattis/cockroach