[Rd] Incomplete valgrind instrumentation level documentation

[Martin Morgan]

The 'Writing R Extensions' manual, configure.ac, and the comment at
the top of memory.c suggests that there are valgrind instrumentation
levels 0, 1, 2 but I think the actual code has four levels.

   level 0 is no additional instrumentation

   level 1 marks uninitialized numeric, logical, integer vectors
           and R_alloc memory

   level 2 marks free memory (DATAPTR, STRING_PTR) and the protection
           stack as inaccessible

   level 3 marks SEXP as inaccessible
-- 
Martin Morgan
Bioconductor / Computational Biology
http://bioconductor.org