recv.2: Improve style in RETURN VALUES

Reported by: des
MFC after: 3 days
Fixes: 571df2c64a3c1 (Explain how recv functions can)
Reviewed by: mhorne, des
Approved by: mhorne (mentor)
Differential Revision: https://reviews.freebsd.org/D48995

(cherry picked from commit 7c3c8605d62f5d27a5228a3787aae6e573f77d59)