docs: add html_root_url attribute

This doc attribute is used by rustdoc when generating documentation
for other crates that depend on this crate. With the html_root_url,
rustdoc will be able to generate correct links into this crate.

See C-HTML-ROOT in the Rust API Guidelines for more information:
https://rust-lang-nursery.github.io/api-guidelines/documentation.html#crate-sets-html_root_url-attribute-c-html-root

A version-sync check was added to ensure that the URL is kept up to
date when the crate version changes.

src/lib.rs

@@ -511,6 +511,7 @@
 //! this repository for more information.
 
 #![crate_type= "lib"]
+#![doc(html_root_url = "https://docs.rs/clap/2.26.2")]
 #![deny(
         missing_docs,
         missing_debug_implementations,

tests/version-numbers.rs

@@ -5,3 +5,8 @@ extern crate version_sync;
 fn test_readme_deps() {
     assert_markdown_deps_updated!("README.md");
 }
+
+#[test]
+fn test_html_root_url() {
+    assert_html_root_url_updated!("src/lib.rs");
+}