Improve wording.

Author: crlf0710

text/0809-box-and-in-for-stdlib.md

@@ -3,7 +3,7 @@
 - RFC PR: [rust-lang/rfcs#809](https://github.com/rust-lang/rfcs/pull/809)
 - Rust Issue: [rust-lang/rust#22181](https://github.com/rust-lang/rust/issues/22181)
 
-# This RFC has been **unapproved**
+# This RFC was previously approved, but later **withdrawn**
 
 For details see the [summary comment].
 

text/1228-placement-left-arrow.md

@@ -3,7 +3,7 @@
 - RFC PR: [rust-lang/rfcs#1228](https://github.com/rust-lang/rfcs/pull/1228)
 - Rust Issue: [rust-lang/rust#27779](https://github.com/rust-lang/rust/issues/27779)
 
-# This RFC has been **unapproved**
+# This RFC was previously approved, but later **withdrawn**
 
 For details see the [summary comment].
 

text/2500-needle.md

@@ -3,7 +3,7 @@
 - RFC PR: [rust-lang/rfcs#2500](https://github.com/rust-lang/rfcs/pull/2500)
 - Rust Issue: [rust-lang/rust#56345](https://github.com/rust-lang/rust/issues/56345)
 
-# This RFC has been **unapproved**
+# This RFC was previously approved, but later **withdrawn**
 
 For details see the [summary comment].