Merge pull request #39732 from sandip-mane/38855-habtm-optional-docs

Adds documentation for habtm association always being optional: true [ci skip]

guides/source/association_basics.md

@@ -118,7 +118,7 @@ end
 ```
 
 When used alone, `belongs_to` produces a one-directional one-to-one connection. Therefore each book in the above example "knows" its author, but the authors don't know about their books.
-To setup a [bi-directional association](#bi-directional-associations) - use `belongs_to` in combination with a `has_one` or `has_many` on the other model. 
+To setup a [bi-directional association](#bi-directional-associations) - use `belongs_to` in combination with a `has_one` or `has_many` on the other model.
 
 `belongs_to` does not ensure reference consistency, so depending on the use case, you might also need to add a database-level foreign key constraint on the reference column, like this:
 
@@ -356,7 +356,9 @@ end
 
 ### The `has_and_belongs_to_many` Association
 
-A `has_and_belongs_to_many` association creates a direct many-to-many connection with another model, with no intervening model. For example, if your application includes assemblies and parts, with each assembly having many parts and each part appearing in many assemblies, you could declare the models this way:
+A `has_and_belongs_to_many` association creates a direct many-to-many connection with another model, with no intervening model.
+This association indicates that each instance of the declaring model refers to zero or more instances of another model.
+For example, if your application includes assemblies and parts, with each assembly having many parts and each part appearing in many assemblies, you could declare the models this way:
 
 ```ruby
 class Assembly < ApplicationRecord