Replace 'Plone package (not add-on)' by 'Plone Core package'

Author: ksuess

docs/install/source-step-by-step.md

@@ -242,7 +242,7 @@ If you want to develop a Plone package, then the subsequent section is for you.
 
 #### Checkout or version pinning of a Plone package
 
-If you want to checkout a Plone package (not add-on) for development or just want to override the constraints of Plone, then a first attempt would be to define constraints with a {file}`constraints.txt` to tell pip to install a different version of a Plone package.
+If you want to checkout a Plone Core package for development or just want to override the constraints of Plone, then a first attempt would be to define constraints with a {file}`constraints.txt` to tell pip to install a different version of a Plone package.
 
 ```
 # constraints.txt with unresolvable version conflict

docs/install/source.md

@@ -265,7 +265,7 @@ Pin the version of a Plone package / constraints
 
 
 Checkout a Plone package
-: If you want to checkout a Plone package (not add-on) for development, this section is for you.
+: If you want to checkout a Plone Core package for development, this section is for you.
 
   Add the Plone package you want to check out in {file}`mx.ini`.