Clarify that atom:content and atom:summary elements are semantically distinct. In particular, content should not be abbreviated.
Implementers (eg WordPress pre 1.5.1) are creating feeds in which both atom:content and atom:summary are set to the same abbreviated version of the post, propagating the loss of distinction in the RSS description element which prevents feed readers (both human and machine) from telling complete from incomplete entries.
In 4.1.3, before the examples change:
The "atom:content" element either contains or links to the content of the entry. The content of atom:content is language-sensitive.
The "atom:content" element either contains or links to the content of the entry. The content of atom:content is language-sensitive. The content of atom:content MUST be a complete representation of the entry. If you wish to create a headlines-only, summaries or extracts feed, you MUST NOT use atom:content, but instead omit it and use atom:summary instead.
This codifies an existing best practice to avoid diluting a key distinction between Atom and predecessor feed formats.