multi-methods need to be explicitly enabled (#10856)

* multi-methods need to be explicitly enabled

* update changelog, manual and tutorial

doc/advopt.txt

@@ -13,7 +13,7 @@ Advanced commands:
   //genDepend               generate a DOT file containing the
                           module dependency graph
   //dump                    dump all defined conditionals and search paths
-                            see also: --dump.format:json (useful with: ` | jq`)
+                          see also: --dump.format:json (useful with: ` | jq`)
   //check                   checks the project for syntax and semantic
 
 Runtime checks (see -x):
@@ -78,7 +78,8 @@ Advanced options:
   --tlsEmulation:on|off     turn thread local storage emulation on|off
   --taintMode:on|off        turn taint mode on|off
   --implicitStatic:on|off   turn implicit compile time evaluation on|off
-  --patterns:on|off         turn pattern matching on|off
+  --patterns:on|off         turn term rewriting macros on|off
+  --multimethods:on|off     turn multi-methods on|off
   --memTracker:on|off       turn memory tracker on|off
   --hotCodeReloading:on|off
                             turn support for hot code reloading on|off

doc/manual.rst

@@ -3716,6 +3716,9 @@ The ``[]`` subscript operator for arrays/openarrays/sequences can be overloaded.
 Multi-methods
 =============
 
+**Note:** Starting from Nim 0.20, to use multi-methods one must explicitly pass
+``--multimethods:on`` when compiling.
+
 Procedures always use static dispatch. Multi-methods use dynamic
 dispatch. For dynamic dispatch to work on an object it should be a reference
 type as well.
@@ -6222,7 +6225,7 @@ Predicate                Meaning
 ===================      =====================================================
 
 Predicates that share their name with a keyword have to be escaped with
-backticks: `` `const` ``.
+backticks.
 The ``alias`` and ``noalias`` predicates refer not only to the matching AST,
 but also to every other bound parameter; syntactically they need to occur after
 the ordinary AST predicates:

doc/tut2.rst

@@ -308,6 +308,9 @@ Note that in the example the constructors ``newLit`` and ``newPlus`` are procs
 because it makes more sense for them to use static binding, but ``eval`` is a
 method because it requires dynamic binding.
 
+**Note:** Starting from Nim 0.20, to use multi-methods one must explicitly pass
+``--multimethods:on`` when compiling.
+
 In a multi-method all parameters that have an object type are used for the
 dispatching: