Mention special matching behaviour of _ for macro_rules!

Author: varkor

src/macros-by-example.md

@@ -140,6 +140,12 @@ the syntax element that matched them. The keyword metavariable `$crate` can be
 used to refer to the current crate; see [Hygiene] below. Metavariables can be
 transcribed more than once or not at all.
 
+For reasons of backwards compatibility, although `_` is a (keyword) identifier,
+it is not matched by the `ident` fragment specifier. Similarly, though `_` [is
+also an expression][_UnderscoreExpression_], a standalone underscore is not
+matched by the `expr` fragment specifier. However, `_` is matched by the `expr`
+fragment specifier when it appears as a subexpression.
+
 ## Repetitions
 
 In both the matcher and transcriber, repetitions are indicated by placing the
@@ -495,6 +501,7 @@ For more detail, see the [formal specification].
 [_Token_]: tokens.md
 [_TypePath_]: paths.md#paths-in-types
 [_Type_]: types.md#type-expressions
+[_UnderscoreExpression_]: expressions/underscore-expr.md
 [_Visibility_]: visibility-and-privacy.md
 [formal specification]: macro-ambiguity.md
 [token]: tokens.md