[Doc] Document void return type for XCConfig methods

Some of these might not actually return nothing, but none of these methods give a guarantee to return a certain result.

lib/cocoapods/generator/xcconfig/xcconfig_helper.rb

@@ -56,6 +56,8 @@ module Pod
         # @param  [Xcodeproj::Config] xcconfig
         #         The xcconfig to edit.
         #
+        # @return [void]
+        #
         def self.add_settings_for_file_accessors_of_target(target, xcconfig)
           target.file_accessors.each do |file_accessor|
             XCConfigHelper.add_spec_build_settings_to_xcconfig(file_accessor.spec_consumer, xcconfig)
@@ -80,6 +82,8 @@ module Pod
         # @param [Spec::FileAccessor] file_accessor
         #        The file accessor, which holds the list of static frameworks.
         #
+        # @return [void]
+        #
         def self.add_static_dependency_build_settings(target, xcconfig, file_accessor)
           file_accessor.vendored_static_frameworks.each do |vendored_static_framework|
             XCConfigHelper.add_framework_build_settings(vendored_static_framework, xcconfig, target.sandbox.root)
@@ -97,6 +101,8 @@ module Pod
         # @param [Xcodeproj::Config] xcconfig
         #        The xcconfig to edit.
         #
+        # @return [void]
+        #
         def self.add_dynamic_dependency_build_settings(target, xcconfig)
           target.file_accessors.each do |file_accessor|
             file_accessor.vendored_dynamic_frameworks.each do |vendored_dynamic_framework|
@@ -117,6 +123,8 @@ module Pod
         # @param  [Xcodeproj::Config] xcconfig
         #         The xcconfig to edit.
         #
+        # @return [void]
+        #
         def self.add_spec_build_settings_to_xcconfig(consumer, xcconfig)
           xcconfig.libraries.merge(consumer.libraries)
           xcconfig.frameworks.merge(consumer.frameworks)
@@ -136,6 +144,8 @@ module Pod
         # @param  [Pathname] sandbox_root
         #         The path retrieved from Sandbox#root.
         #
+        # @return [void]
+        #
         def self.add_framework_build_settings(framework_path, xcconfig, sandbox_root)
           name = File.basename(framework_path, '.framework')
           dirname = '${PODS_ROOT}/' + framework_path.dirname.relative_path_from(sandbox_root).to_s
@@ -158,6 +168,8 @@ module Pod
         # @param  [Pathname] sandbox_root
         #         The path retrieved from Sandbox#root.
         #
+        # @return [void]
+        #
         def self.add_library_build_settings(library_path, xcconfig, sandbox_root)
           extension = File.extname(library_path)
           name = File.basename(library_path, extension).sub(/\Alib/, '')
@@ -179,6 +191,8 @@ module Pod
         # @param  [Xcodeproj::Config] xcconfig
         #         The xcconfig to edit.
         #
+        # @return [void]
+        #
         def self.add_code_signing_settings(target, xcconfig)
           build_settings = {}
           if target.platform.to_sym == :osx
@@ -196,6 +210,8 @@ module Pod
         # @param  [Xcodeproj::Config] xcconfig
         #         The xcconfig to edit.
         #
+        # @return [void]
+        #
         def self.add_target_specific_settings(target, xcconfig)
           if target.requires_frameworks?
             add_code_signing_settings(target, xcconfig)
@@ -215,6 +231,8 @@ module Pod
         # @param  [Xcodeproj::Config] xcconfig
         #         The xcconfig to edit.
         #
+        # @return [void]
+        #
         def self.add_settings_for_dependent_targets(target, dependent_targets, xcconfig)
           dependent_targets = dependent_targets.select(&:should_build?)
           has_configuration_build_dir = target.respond_to?(:configuration_build_dir)
@@ -253,6 +271,8 @@ module Pod
         # @param  [Xcodeproj::Config] xcconfig
         #         The xcconfig to edit.
         #
+        # @return [void]
+        #
         def self.add_language_specific_settings(target, xcconfig)
           if target.uses_swift?
             build_settings = {