Add note and link to the issue

Author: 0xPoe

doc/src/overrides.md

@@ -22,6 +22,7 @@ override that is further away.
 To verify which toolchain is active, you can use `rustup show`, 
 which will also try to install the corresponding
 toolchain if the current one has not been installed according to the above rules.
+(Please note that this behavior is subject to change, as detailed in issue [#1397].)
 
 [toolchain]: concepts/toolchains.md
 [toolchain override shorthand]: #toolchain-override-shorthand
@@ -143,3 +144,4 @@ rustup default nightly-2020-07-27
 ```
 
 [installed]: installation/index.md
+[#1397]: https://github.com/rust-lang/rustup/issues/1397