Clarify conditional compilation

Introduction of `cfg` attribute and `cfg!` macro operators is
misleading. `cfg!` macro has nothing to do with conditional compilation,
but it can be used for run-time conditional checks. It is `cfg`
attribute that enables conditional compilation. Clarify the
responsibilities of the operators.

src/attribute/cfg.md

@@ -1,11 +1,12 @@
 # `cfg`
 
-Conditional compilation is possible through two different operators:
+Configuration conditional checks are possible through two different operators:
 
 * the `cfg` attribute: `#[cfg(...)]` in attribute position
 * the `cfg!` macro: `cfg!(...)` in boolean expressions
 
-Both utilize identical argument syntax.
+While the former enables conditional compilation, the latter allows for run-time
+checks. Both utilize identical argument syntax.
 
 ```rust,editable
 // This function only gets compiled if the target OS is linux
@@ -22,7 +23,7 @@ fn are_you_on_linux() {
 
 fn main() {
     are_you_on_linux();
-    
+
     println!("Are you sure?");
     if cfg!(target_os = "linux") {
         println!("Yes. It's definitely linux!");