Fixed #22029 -- Removed obsolete advice on registering migrate/syncdb signal handlers.

All signals should now be registered in AppConfig.ready().

Thanks un33k for the report.

docs/ref/signals.txt

@@ -376,11 +376,6 @@ pre_migrate
 Sent by the :djadmin:`migrate` command before it starts to install an
 application. It's not emitted for applications that lack a ``models`` module.
 
-Any handlers that listen to this signal need to be written in a particular
-place: a ``management`` module in one of your :setting:`INSTALLED_APPS`. If
-handlers are registered anywhere else they may not be loaded by
-:djadmin:`migrate`.
-
 Arguments sent with this signal:
 
 ``sender``
@@ -421,11 +416,6 @@ pre_syncdb
 Sent by the :djadmin:`syncdb` command before it starts to install an
 application.
 
-Any handlers that listen to this signal need to be written in a particular
-place: a ``management`` module in one of your :setting:`INSTALLED_APPS`. If
-handlers are registered anywhere else they may not be loaded by
-:djadmin:`syncdb`.
-
 Arguments sent with this signal:
 
 ``sender``
@@ -469,13 +459,9 @@ Sent by the :djadmin:`migrate` command after it installs an application, and the
 :djadmin:`flush` command. It's not emitted for applications that lack a
 ``models`` module.
 
-Any handlers that listen to this signal need to be written in a particular
-place: a ``management`` module in one of your :setting:`INSTALLED_APPS`. If
-handlers are registered anywhere else they may not be loaded by
-:djadmin:`migrate`. It is important that handlers of this signal perform
-idempotent changes (e.g. no database alterations) as this may cause the
-:djadmin:`flush` management command to fail if it also ran during the
-:djadmin:`migrate` command.
+It is important that handlers of this signal perform idempotent changes (e.g.
+no database alterations) as this may cause the :djadmin:`flush` management
+command to fail if it also ran during the :djadmin:`migrate` command.
 
 Arguments sent with this signal:
 
@@ -529,13 +515,9 @@ post_syncdb
 Sent by the :djadmin:`syncdb` command after it installs an application, and the
 :djadmin:`flush` command.
 
-Any handlers that listen to this signal need to be written in a particular
-place: a ``management`` module in one of your :setting:`INSTALLED_APPS`. If
-handlers are registered anywhere else they may not be loaded by
-:djadmin:`syncdb`. It is important that handlers of this signal perform
-idempotent changes (e.g. no database alterations) as this may cause the
-:djadmin:`flush` management command to fail if it also ran during the
-:djadmin:`syncdb` command.
+It is important that handlers of this signal perform idempotent changes (e.g.
+no database alterations) as this may cause the :djadmin:`flush` management
+command to fail if it also ran during the :djadmin:`syncdb` command.
 
 Arguments sent with this signal: