Update documentation wrt new chunk size cap

src/vmod_slash.vcc

@@ -483,13 +483,16 @@ fellow storage can be fine tuned:
   - unit: bytes as a power of two / bytes
   - default: 20 / 1 MB
   - minimum: 12 / 4 KB
-  - maximum: 28 / 256 MB
+  - maximum: 28 / 256 MB or <1/1024 of memsize
 
   *chunk_bytes* and *chunk_exponent* are alternative ways to configure
   the chunk size. If *chunk_bytes* is used, the value is rounded up to
   the next power of two and used as if *chunk_exponent* was used with
   the 2-logarithm of that value.
 
+  *chunk_bytes* / *chunk_exponent* are hard capped to less than 1/1024
+  of the memory cache size.
+
   Using both arguments at the same time triggers a VCL error.
 
   See `xbuddy.tune()` for additional details.