Add option style comment for rustc_optgroups

Signed-off-by: xizheyin <[email protected]>

Author: xizheyin

compiler/rustc_session/src/config.rs

@@ -1665,6 +1665,13 @@ static EMIT_HELP: LazyLock<String> = LazyLock::new(|| {
 
 /// Returns all rustc command line options, including metadata for
 /// each option, such as whether the option is stable.
+///
+/// # Option style guidelines
+///
+/// - `<param>`: Indicates a required parameter
+/// - `[param]`: Indicates an optional parameter
+/// - `|`: Indicates a mutually exclusive option
+/// - `*`: a list element with description
 pub fn rustc_optgroups() -> Vec<RustcOptGroup> {
     use OptionKind::{Flag, FlagMulti, Multi, Opt};
     use OptionStability::{Stable, Unstable};