apply suggestion on the use of statics in const-eval

Author: dingxiangfei2009

src/const_eval.md

@@ -23,9 +23,10 @@ to be run.
 * [Const parameters].
 * [Paths] to [functions] and [constants].
   Recursively defining constants is not allowed.
-* Paths to immutable [statics].
-    * Reads of [`extern` statics] are not allowed.
-    * Reads from and writes to a `static` with interior mutability are not allowed.
+* Paths to immutable [statics] with these restrictions and observations.
+    * In particular, reads and writes to `static mut` are not allowed.
+    * Reads out of and writes into a `static` with data equipped with interior mutability are not allowed.
+    * Reads out of and writes into [`extern` statics] are not allowed.
 * [Tuple expressions].
 * [Array expressions].
 * [Struct] expressions.