Doc: clarify a critical and undocumented aspect of simplehash.h.

I just got burnt by trying to use pg_malloc instead of pg_malloc0
with this.  Save the next hacker some time by not leaving this
API detail undocumented.

src/include/lib/simplehash.h

@@ -41,7 +41,7 @@
  *	  - SH_SCOPE - in which scope (e.g. extern, static inline) do function
  *		declarations reside
  *	  - SH_RAW_ALLOCATOR - if defined, memory contexts are not used; instead,
- *	    use this to allocate bytes
+ *	    use this to allocate bytes. The allocator must zero the returned space.
  *	  - SH_USE_NONDEFAULT_ALLOCATOR - if defined no element allocator functions
  *		are defined, so you can supply your own
  *	  The following parameters are only relevant when SH_DEFINE is defined: