[components][expression_language] Add doc for backslashes

Additional backslashes are required to escape a backslash(``\``)
in a string or regex because a string will be stripped by the lexer.
This should be documented here, otherwise, user may feel confused about
the unexpected behavior.

| Q             | A
| ------------- | ---
| Doc fix?      | yes
| New docs?     | no
| Applies to    | ~2.4
| Fixed tickets | n/a

Author: zerustech

components/expression_language/syntax.rst

@@ -181,6 +181,19 @@ Comparison Operators
     You must use parenthesis because the unary operator ``not`` has precedence
     over the binary operator ``matches``.
 
+    A backslash(\) must be escaped by 4 backslashes(``\\\\``) in a string and
+    8 backslashes(``\\\\\\\\``) in a regex::
+
+        $language->evaluate('"\\\\"');
+        // returns \
+
+        $language->evaluate('"a\\\\b" matches "/^a\\\\\\\\b$/"');
+        // returns true
+
+    This is because the backslashes in a string will be stripped by the
+    ``stripcslashes()`` function and the stripped slashes in a regex will be
+    stripped again by the regex engine.
+
 Examples::
 
     $ret1 = $language->evaluate(