update cfg_select! documentation

Author: folkertdev

library/core/src/macros/mod.rs

@@ -198,14 +198,11 @@ pub macro assert_matches {
 
 /// A macro for defining `#[cfg]` match-like statements.
 ///
-/// It is similar to the `if/elif` C preprocessor macro by allowing definition of a cascade of
-/// `#[cfg]` cases, emitting the implementation which matches first.
+/// The `cfg_select!` macro checks a series of `#[cfg]` conditions, and picks the branch that
+/// belongs to the first condition that evaluates to true.
 ///
-/// This allows you to conveniently provide a long list `#[cfg]`'d blocks of code
-/// without having to rewrite each clause multiple times.
-///
-/// Trailing `_` wildcard match arms are **optional** and they indicate a fallback branch when
-/// all previous declarations do not evaluate to true.
+/// An optional trailing `_` wildcard match arm can be used to specify a fallback branch.
+/// If none of the conditions evaluate to `true`, a compile error is emitted.
 ///
 /// # Example
 ///
@@ -225,7 +222,7 @@ pub macro assert_matches {
 /// }
 /// ```
 ///
-/// If desired, it is possible to return expressions through the use of surrounding braces:
+/// The `cfg_select!` macro can also be used in expression position:
 ///
 /// ```
 /// #![feature(cfg_select)]

tests/ui/macros/cfg_select.rs

@@ -25,3 +25,6 @@ cfg_select! {
     //~^ ERROR none of the rules in this `cfg_select` evaluated to true
     false => {}
 }
+
+cfg_select! {}
+//~^ ERROR none of the rules in this `cfg_select` evaluated to true

tests/ui/macros/cfg_select.stderr

@@ -21,5 +21,11 @@ LL | |     false => {}
 LL | | }
    | |_^
 
-error: aborting due to 2 previous errors; 1 warning emitted
+error: none of the rules in this `cfg_select` evaluated to true
+  --> $DIR/cfg_select.rs:29:1
+   |
+LL | cfg_select! {}
+   | ^^^^^^^^^^^^^^
+
+error: aborting due to 3 previous errors; 1 warning emitted