Rollup merge of rust-lang#81968 - pietroalbini:fix-doc-install-path, …

…r=Mark-Simulacrum

bootstrap: fix wrong docs installation path

This PR fixes rust-lang#81967, a regression introduced by rust-lang#80797. The commit has already been backported to stable 1.50.0.

r? `````@Mark-Simulacrum`````

src/bootstrap/install.rs

@@ -29,7 +29,7 @@ fn install_sh(
     let prefix = default_path(&builder.config.prefix, "/usr/local");
     let sysconfdir = prefix.join(default_path(&builder.config.sysconfdir, "/etc"));
     let datadir = prefix.join(default_path(&builder.config.datadir, "share"));
-    let docdir = prefix.join(default_path(&builder.config.docdir, "share/doc"));
+    let docdir = prefix.join(default_path(&builder.config.docdir, "share/doc/rust"));
     let mandir = prefix.join(default_path(&builder.config.mandir, "share/man"));
     let libdir = prefix.join(default_path(&builder.config.libdir, "lib"));
     let bindir = prefix.join(&builder.config.bindir); // Default in config.rs