Rollup merge of #79464 - GuillaumeGomez:doc-keyword-ident, r=jyn514

Extend doc keyword feature by allowing any ident

Part of #51315.

As suggested by ``@danielhenrymantilla`` in [this comment](https://github.com/rust-lang/rust/issues/51315#issuecomment-733879934), this PR extends `#[doc(keyword = "...")]` to allow any ident to be used as keyword. The final goal is to allow (proc-)macro crates' owners to write documentation of the keywords they might introduce.

r? ``@jyn514``

compiler/rustc_span/src/symbol.rs

@@ -1590,11 +1590,6 @@ impl Symbol {
         self == kw::Try
     }
 
-    /// Used for sanity checking rustdoc keyword sections.
-    pub fn is_doc_keyword(self) -> bool {
-        self <= kw::Union
-    }
-
     /// A keyword or reserved identifier that can be used as a path segment.
     pub fn is_path_segment_keyword(self) -> bool {
         self == kw::Super