Update -Zmiri-enable-communication docs

Author: pvdrz

README.md

@@ -158,8 +158,8 @@ Several `-Z` flags are relevant for Miri:
   will miss bugs in your program.  However, this can also help to make Miri run
   faster.
 * `-Zmiri-enable-communication` enables communication between the host
-  environment and Miri, i.e., all the host environment variables are available
-  during Miri runtime.
+  environment and Miri, i.e., Miri uses the host's random number generator and 
+  all the host environment variables are available during runtime.
 * `-Zmir-opt-level` controls how many MIR optimizations are performed.  Miri
   overrides the default to be `0`; be advised that using any higher level can
   make Miri miss bugs in your program because they got optimized away.

src/machine.rs

@@ -93,7 +93,8 @@ pub struct Evaluator<'tcx> {
     /// TLS state.
     pub(crate) tls: TlsData<'tcx>,
 
-    /// If enabled, the `env_vars` field is populated with the host env vars during initialization.
+    /// If enabled, the `env_vars` field is populated with the host env vars during initialization
+    /// and random number generation is delegated to the host.
     pub(crate) communicate: bool,
 }