Rollup merge of rust-lang#37757 - rust-lang:E0002-precision, r=brson

Uncomment some long error explanation

Retry of rust-lang#37058.

r? @steveklabnik
cc @brson

Author: GuillaumeGomez

src/librustc_const_eval/diagnostics.rs

@@ -40,7 +40,9 @@ Ensure the ordering of the match arm is correct and remove any superfluous
 arms.
 "##,
 
-/*E0002: r##"
+E0002: r##"
+## Note: this error code is no longer emitted by the compiler.
+
 This error indicates that an empty match expression is invalid because the type
 it is matching on is non-empty (there exist values of this type). In safe code
 it is impossible to create an instance of an empty type, so empty match
@@ -68,10 +70,11 @@ fn foo(x: Option<String>) {
     }
 }
 ```
-"##,*/
+"##,
 
+E0003: r##"
+## Note: this error code is no longer emitted by the compiler.
 
-/*E0003: r##"
 Not-a-Number (NaN) values cannot be compared for equality and hence can never
 match the input to a match expression. So, the following will not compile:
 
@@ -98,7 +101,6 @@ match number {
 }
 ```
 "##,
-*/
 
 E0004: r##"
 This error indicates that the compiler cannot guarantee a matching pattern for