Teach rustdoc how to display WASI.

As a followup to [this comment] in rust-lang#82420, this patch teaches rustdoc
how to display WASI.

[this comment]: rust-lang#82420 (comment)

Author: sunfishcode

src/librustdoc/clean/cfg.rs

@@ -483,6 +483,7 @@ impl<'a> fmt::Display for Display<'a> {
                         "openbsd" => "OpenBSD",
                         "redox" => "Redox",
                         "solaris" => "Solaris",
+                        "wasi" => "WASI",
                         "windows" => "Windows",
                         _ => "",
                     },

src/librustdoc/clean/cfg/tests.rs

@@ -367,6 +367,10 @@ fn test_render_long_html() {
             name_value_cfg("target_os", "macos").render_long_html(),
             "This is supported on <strong>macOS</strong> only."
         );
+        assert_eq!(
+            name_value_cfg("target_os", "wasi").render_long_html(),
+            "This is supported on <strong>WASI</strong> only."
+        );
         assert_eq!(
             name_value_cfg("target_pointer_width", "16").render_long_html(),
             "This is supported on <strong>16-bit</strong> only."