document the keywords _ident behavior

docs/language-guide.rst

@@ -87,12 +87,14 @@ object, each property of which defines its own group of keywords:
   {
     keywords: {
       keyword: 'else for if while',
-      literal: ['false','true','null']
+      literal: ['false','true','null'],
+      _relevance_only: 'one two three four'
     }
   }
 
 The group name becomes the class name in the generated markup, enabling different
-theming for different kinds of keywords.
+theming for different kinds of keywords.  Any property starting with a ``_`` will
+only use those keywords to increase relevance, they will not be highlighted.
 
 To detect keywords, highlight.js breaks the processed chunk of code into separate
 words — a process called lexing. By default, "words" are matched with the regexp

src/highlight.js

@@ -172,8 +172,9 @@ const HLJS = function(hljs) {
           buf = "";
 
           relevance += keywordRelevance;
-          if (kind === "_") {
-            // _ is magic identifier for relevance only, do not highlight
+          if (kind.startsWith("_")) {
+            // _ implied for relevance only, do not highlight
+            // by applying a class name
             buf += match[0];
           } else {
             const cssClass = language.classNameAliases[kind] || kind;

src/languages/cpp.js

@@ -237,8 +237,7 @@ export default function(hljs) {
       'atomic_ullong new throw return ' +
       'and and_eq bitand bitor compl not not_eq or or_eq xor xor_eq',
     built_in: '_Bool _Complex _Imaginary',
-    // used only for relevance
-    _: COMMON_CPP_HINTS,
+    _relevance_hints: COMMON_CPP_HINTS,
     literal: 'true false nullptr NULL'
   };