Clarify validations that use validate_change internally (#4509)

lib/ecto/changeset.ex

@@ -2827,6 +2827,9 @@ defmodule Ecto.Changeset do
 
   The format has to be expressed as a regular expression.
 
+  The validation only runs if a change for the given `field` exists and the
+  change value is not `nil`.
+
   ## Options
 
     * `:message` - the message on failure, defaults to "has invalid format".
@@ -2855,6 +2858,9 @@ defmodule Ecto.Changeset do
   @doc """
   Validates a change is included in the given enumerable.
 
+  The validation only runs if a change for the given `field` exists and the
+  change value is not `nil`.
+
   ## Options
 
     * `:message` - the message on failure, defaults to "is invalid".
@@ -2887,6 +2893,9 @@ defmodule Ecto.Changeset do
 
   Type of the field must be array.
 
+  The validation only runs if a change for the given `field` exists and the
+  change value is not `nil`.
+
   ## Options
 
     * `:message` - the message on failure, defaults to "has an invalid entry".
@@ -2926,6 +2935,9 @@ defmodule Ecto.Changeset do
   @doc """
   Validates a change is not included in the given enumerable.
 
+  The validation only runs if a change for the given `field` exists and the
+  change value is not `nil`.
+
   ## Options
 
     * `:message` - the message on failure, defaults to "is reserved".
@@ -2957,6 +2969,9 @@ defmodule Ecto.Changeset do
   of unicode characters. Then consider using the `:count` option to
   limit the number of codepoints (`:codepoints`), or limit the number of bytes (`:bytes`).
 
+  The validation only runs if a change for the given `field` exists and the
+  change value is not `nil`.
+
   ## Options
 
     * `:is` - the length must be exactly this value
@@ -3153,6 +3168,9 @@ defmodule Ecto.Changeset do
   @doc """
   Validates the properties of a number.
 
+  The validation only runs if a change for the given `field` exists and the
+  change value is not `nil`.
+
   ## Options
 
     * `:less_than`