[Rustc Book] Explain --cfg's arguments

Author: Havvy

src/doc/rustc/src/command-line-arguments.md

@@ -10,6 +10,11 @@ This flag will print out help information for `rustc`.
 
 This flag can turn on or off various `#[cfg]` settings.
 
+The value can either be a single identifier or two identifiers separated by `=`.
+
+For examples, `--cfg 'verbose'` or `--cfg 'feature=serde'`. These correspond
+to `#[cfg(verbose)]` and `#[cfg(feature = "serde")]` respectively.
+
 ## `-L`: add a directory to the library search path
 
 When looking for external crates, a directory passed to this flag will be searched.