lint-docs: Add some extra detail to the error message.

This will hopefully help users figure out what was wrong and how
to fix it.

src/tools/lint-docs/src/main.rs

@@ -4,6 +4,19 @@ use std::path::PathBuf;
 fn main() {
     if let Err(e) = doit() {
         eprintln!("error: {}", e);
+        eprintln!(
+            "
+This error was generated by the lint-docs tool.
+This tool extracts documentation for lints from the source code and places
+them in the rustc book. See the declare_lint! documentation
+https://doc.rust-lang.org/nightly/nightly-rustc/rustc_lint_defs/macro.declare_lint.html
+for an example of the format of documentation this tool expects.
+
+To re-run these tests, run: ./x.py test --keep-stage=0 src/tools/lint-docs
+The --keep-stage flag should be used if you have already built the compiler
+and are only modifying the doc comments to avoid rebuilding the compiler.
+"
+        );
         std::process::exit(1);
     }
 }