nfsd.8: Fix mandoc -Tlint and igor warnings

• Remove Tn macros
• Refernce sysctl(8) instead of sysctl(1)
• Start new sentences on new lines
• Capitalize NFS where needed
• Use Fx for FreeBSD
• Remove a list block (Bl) that was added to the manual page by accident in r335174

Reviewed by: bcr
Approved by: doc (bcr)
Differential Revision: https://reviews.freebsd.org/D20215