diff --git a/actionpack/lib/action_controller/metal/head.rb b/actionpack/lib/action_controller/metal/head.rb
index b2110bf946..5e9832fd4e 100644
--- a/actionpack/lib/action_controller/metal/head.rb
+++ b/actionpack/lib/action_controller/metal/head.rb
@@ -50,7 +50,6 @@ def head(status, options = {})
     end
 
     private
-    # :nodoc:
     def include_content?(status)
       case status
       when 100..199
diff --git a/activesupport/lib/active_support/core_ext/hash/conversions.rb b/activesupport/lib/active_support/core_ext/hash/conversions.rb
index 8594d9bf2e..6741e732f0 100644
--- a/activesupport/lib/active_support/core_ext/hash/conversions.rb
+++ b/activesupport/lib/active_support/core_ext/hash/conversions.rb
@@ -138,6 +138,8 @@ def from_trusted_xml(xml)
 
 module ActiveSupport
   class XMLConverter # :nodoc:
+    # Raised if the XML contains attributes with type="yaml" or
+    # type="symbol". Read Hash#from_xml for more details.
     class DisallowedType < StandardError
       def initialize(type)
         super "Disallowed type attribute: #{type.inspect}"
diff --git a/activesupport/lib/active_support/core_ext/string/output_safety.rb b/activesupport/lib/active_support/core_ext/string/output_safety.rb
index 04ed8e7cd8..6251f34daf 100644
--- a/activesupport/lib/active_support/core_ext/string/output_safety.rb
+++ b/activesupport/lib/active_support/core_ext/string/output_safety.rb
@@ -141,6 +141,7 @@ class SafeBuffer < String
     alias_method :original_concat, :concat
     private :original_concat
 
+    # Raised when <tt>ActiveSupport::SafeBuffer#safe_concat</tt> is called on unsafe buffers.
     class SafeConcatError < StandardError
       def initialize
         super 'Could not concatenate to the buffer because it is not html safe.'
diff --git a/activesupport/lib/active_support/deprecation/behaviors.rb b/activesupport/lib/active_support/deprecation/behaviors.rb
index 28d2d78643..0de891f1a2 100644
--- a/activesupport/lib/active_support/deprecation/behaviors.rb
+++ b/activesupport/lib/active_support/deprecation/behaviors.rb
@@ -1,6 +1,8 @@
 require "active_support/notifications"
 
 module ActiveSupport
+  # Raised when <tt>ActiveSupport::Deprecation::Behavior#behavior</tt> is set with <tt>:raise</tt>.
+  # You would set <tt>:raise</tt>, as a behaviour to raise errors and proactively report exceptions from deprecations.
   class DeprecationException < StandardError
   end