rewording and make different cases for static and const eval context

Author: dingxiangfei2009

src/const_eval.md

@@ -43,7 +43,8 @@ r[const-eval.const-expr.path-item]
 
 r[const-eval.const-expr.path-static]
 * Paths to [statics] with these restrictions and observations.
-  * In particular, reads and writes to any `static mut` or [`extern` statics] is not allowed.
+  * If the evaluation is carried out in an initializer of a `const` item, or inside a `const` block, reads and writes to any `static mut` or [`extern` statics] are not allowed.
+  * If the evaluation is carried out in an initializer of a `static` item, reads into a `static`, or inside `unsafe` blocks reads into `static mut` are allowed. Reads and writes to `extern` statics are still not allowed.
 
 r[const-eval.const-expr.tuple]
 * [Tuple expressions].

src/items/constant-items.md

@@ -48,6 +48,7 @@ const BITS_N_STRINGS: BitsNStrings<'static> = BitsNStrings {
 };
 ```
 
+r[items.const.final-value-immutable]
 The final value of a `const` item cannot contain references to anything mutable.
 
 r[items.const.expr-omission]