Explicitly comment why we set watcher_info struct's max_size manually
Add missing netsnmp_watcher_info fields definition to netsnmpapi

Basically we do not need fields definitions for net-snmp structures
that we never modify ourselves. However in this case we actually
tried setting netsnmp_watcher_info's "max_size" field which resulted
in a no-op.

Reported by Max "mk23" Kalika <max.kalika+projects@gmail.com>.