docs(compiler): add comment to warn about regexp changes (#14106)

ref #14082

modules/@angular/compiler/src/selector.ts

@@ -12,6 +12,7 @@ const _SELECTOR_REGEXP = new RegExp(
     '(\\:not\\()|' +                               //":not("
         '([-\\w]+)|' +                             // "tag"
         '(?:\\.([-\\w]+))|' +                      // ".class"
+        // "-" should appear first in the regexp below as FF31 parses "[.-\w]" as a range
         '(?:\\[([-.\\w*]+)(?:=([^\\]]*))?\\])|' +  // "[name]", "[name=value]"
         '(\\))|' +                                 // ")"
         '(\\s*,\\s*)',                             // ","