fts: Further improve the manual page

• Add subsections for the three functions that didn't already have one.
• Add a RETURN VALUES section.
• Improve the grammar somewhat.
• Clarify that fts_read() will not set errno to 0 if called again after having already returned NULL.

Sponsored by: Klara, Inc.
Reviewed by: bcr, markj
Differential Revision: https://reviews.freebsd.org/D52925