recv.2: Explain how recv functions can return 0

Clarify the RETURN VALUES section with improved structure,
the condition of the return value 0, and the setting of errno.

PR: 174581
Reviewed by: jhb, ziaee
Approved by: mhorne (mentor)
Differential Revision: https://reviews.freebsd.org/D48955

(cherry picked from commit 571df2c64a3c1af1fe011303ec08e391e887ecbc)