[ci skip] Move association class method notes

Make explanation of association class methods clearer by moving notes to beginning of each example section.

activerecord/lib/active_record/associations.rb

@@ -1034,6 +1034,9 @@ module ClassMethods
       # Specifies a one-to-many association. The following methods for retrieval and query of
       # collections of associated objects will be added:
       #
+      # +collection+ is a placeholder for the symbol passed as the first argument, so
+      # <tt>has_many :clients</tt> would add among others <tt>clients.empty?</tt>.
+      #
       # [collection(force_reload = false)]
       #   Returns an array of all the associated objects.
       #   An empty array is returned if none are found.
@@ -1092,9 +1095,6 @@ module ClassMethods
       #   Does the same as <tt>collection.create</tt>, but raises <tt>ActiveRecord::RecordInvalid</tt>
       #   if the record is invalid.
       #
-      # (*Note*: +collection+ is replaced with the symbol passed as the first argument, so
-      # <tt>has_many :clients</tt> would add among others <tt>clients.empty?</tt>.)
-      #
       # === Example
       #
       # A <tt>Firm</tt> class declares <tt>has_many :clients</tt>, which will add:
@@ -1209,6 +1209,9 @@ def has_many(name, scope = nil, options = {}, &extension)
       #
       # The following methods for retrieval and query of a single associated object will be added:
       #
+      # +association+ is a placeholder for the symbol passed as the first argument, so
+      # <tt>has_one :manager</tt> would add among others <tt>manager.nil?</tt>.
+      #
       # [association(force_reload = false)]
       #   Returns the associated object. +nil+ is returned if none is found.
       # [association=(associate)]
@@ -1226,9 +1229,6 @@ def has_many(name, scope = nil, options = {}, &extension)
       #   Does the same as <tt>create_association</tt>, but raises <tt>ActiveRecord::RecordInvalid</tt>
       #   if the record is invalid.
       #
-      # (+association+ is replaced with the symbol passed as the first argument, so
-      # <tt>has_one :manager</tt> would add among others <tt>manager.nil?</tt>.)
-      #
       # === Example
       #
       # An Account class declares <tt>has_one :beneficiary</tt>, which will add:
@@ -1314,6 +1314,9 @@ def has_one(name, scope = nil, options = {})
       # Methods will be added for retrieval and query for a single associated object, for which
       # this object holds an id:
       #
+      # +association+ is a placeholder for the symbol passed as the first argument, so
+      # <tt>belongs_to :author</tt> would add among others <tt>author.nil?</tt>.
+      #
       # [association(force_reload = false)]
       #   Returns the associated object. +nil+ is returned if none is found.
       # [association=(associate)]
@@ -1329,9 +1332,6 @@ def has_one(name, scope = nil, options = {})
       #   Does the same as <tt>create_association</tt>, but raises <tt>ActiveRecord::RecordInvalid</tt>
       #   if the record is invalid.
       #
-      # (+association+ is replaced with the symbol passed as the first argument, so
-      # <tt>belongs_to :author</tt> would add among others <tt>author.nil?</tt>.)
-      #
       # === Example
       #
       # A Post class declares <tt>belongs_to :author</tt>, which will add:
@@ -1451,6 +1451,9 @@ def belongs_to(name, scope = nil, options = {})
       #
       # Adds the following methods for retrieval and query:
       #
+      # +collection+ is a placeholder for the symbol passed as the first argument, so
+      # <tt>has_and_belongs_to_many :categories</tt> would add among others <tt>categories.empty?</tt>.
+      #
       # [collection(force_reload = false)]
       #   Returns an array of all the associated objects.
       #   An empty array is returned if none are found.
@@ -1492,9 +1495,6 @@ def belongs_to(name, scope = nil, options = {})
       #   with +attributes+, linked to this object through the join table, and that has already been
       #   saved (if it passed the validation).
       #
-      # (+collection+ is replaced with the symbol passed as the first argument, so
-      # <tt>has_and_belongs_to_many :categories</tt> would add among others <tt>categories.empty?</tt>.)
-      #
       # === Example
       #
       # A Developer class declares <tt>has_and_belongs_to_many :projects</tt>, which will add: