Add Method Definitions section

Author: soutaro

docs/inline.md

@@ -111,3 +111,81 @@ This creates the types `::API`, `::API::V1`, and `::API::V1::Resources`.
 - Generic module definitions are not supported
 - Module self-type constraints are not supported
 
+## Method Definitions
+
+Inline RBS supports methods defined using the `def` syntax in Ruby.
+
+```ruby
+class Calculator
+  def add(x, y) = x+y
+end
+```
+
+It detects method definitions and allows you to add annotation comments to describe their types.
+
+### Unannotated method definition
+
+Methods defined with `def` syntax are detected, but they are untyped.
+
+```ruby
+class Calculator
+  def add(x, y) = x+y
+end
+```
+
+The type of the `Calculator#add` method is `(?) -> untyped` -- it accepts any arguments without type checking and returns an `untyped` object.
+
+### Method type annotation syntax
+
+You can define the type of the method using `@rbs` and `:` syntax.
+
+```ruby
+class Calculator
+  # @rbs (Integer, Integer) -> Integer
+  def add(x, y) = x + y
+
+  #: (Integer, Integer) -> Integer
+  def subtract(x, y) = x - y
+end
+```
+
+The type of both methods is `(Integer, Integer) -> Integer` -- they take two `Integer` objects and return an `Integer` object.
+
+Both syntaxes support method overloading:
+
+```ruby
+class Calculator
+  # @rbs (Integer, Integer) -> Integer
+  #    | (Float, Float) -> Float
+  def add(x, y) = x + y
+
+  #: (Integer, Integer) -> Integer
+  #: (Float, Float) -> Float
+  def subtract(x, y) = x - y
+end
+```
+
+The type of both methods is `(Integer, Integer) -> Integer | (Float, Float) -> Float`.
+
+> [!NOTE]
+> The `@rbs METHOD-TYPE` syntax allows overloads with the `|` operator, just like in RBS files.
+> Multiple `: METHOD-TYPE` declarations are required for overloads.
+
+#### Doc-style syntax
+
+The `@rbs return: T` syntax declares the return type of a method:
+
+```ruby
+class Calculator
+  # @rbs return: String
+  def to_s
+    "Calculator"
+  end
+end
+```
+
+### Current Limitations
+
+- Class methods and singleton methods are not supported
+- Parameter types are not supported with doc-style syntax
+- Method visibility declaration is not supported yet