Clean up rustdoc command line args doc

Author: GuillaumeGomez

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

@@ -381,7 +381,7 @@ the same CSS rules as the official `light` theme.
 `--check-theme` flag, it discards all other flags and only performs the CSS rule
 comparison operation.
 
-### `--crate-version`: control the crate version
+## `--crate-version`: control the crate version
 
 Using this flag looks like this:
 
@@ -418,6 +418,8 @@ Rustdoc only supports Rust source code and Markdown input formats. If the
 file ends in `.md` or `.markdown`, `rustdoc` treats it as a Markdown file.
 Otherwise, it assumes that the input file is Rust.
 
+# Unstable command line arguments
+
 ## `--nocapture`
 
 When this flag is used with `--test`, the output (stdout and stderr) of your tests won't be