update README

Author: RalfJung

README.md

@@ -147,8 +147,8 @@ Several `-Z` flags are relevant for Miri:
 
 * `-Zmiri-seed=<hex>` is a custom `-Z` flag added by Miri.  It configures the
   seed of the RNG that Miri uses to resolve non-determinism.  This RNG is used
-  to pick base addresses for allocations, and when the interpreted program
-  requests system entropy.  The default seed is 0.
+  to pick base addresses for allocations.  When isolation is enabled (the default),
+  this is also used to emulate system entropy.  The default seed is 0.
   **NOTE**: This entropy is not good enough for cryptographic use!  Do not
   generate secret keys in Miri or perform other kinds of cryptographic
   operations that rely on proper random numbers.
@@ -157,8 +157,8 @@ Several `-Z` flags are relevant for Miri:
   useful for debugging.  It means Miri will miss bugs in your program.  However,
   this can also help to make Miri run faster.
 * `-Zmiri-disable-isolation` disables host host isolation.  As a consequence,
-  the program has access to host resources such as environment variables and
-  randomness (and, eventually, file systems and more).
+  the program has access to host resources such as environment variables, file
+  systems, and randomness.
 * `-Zmiri-ignore-leaks` disables the memory leak checker.
 * `-Zmiri-env-exclude=<var>` keeps the `var` environment variable isolated from
   the host. Can be used multiple times to exclude several variables. The `TERM`