docs: add BFD local-address and interface documentation for BGP neigh…

[r3m8]

Summary

Documented the new BFD neighbor-level Local Address and Interface options in FRR dynamic routing docs.

What changed

• Updated source/manual/dynamic_routing.rst in the Routing → BFD → Neighbors options table.
• Added Local Address with:
  • behavior: specifies the local IP address to bind the peer listener to and to use for sending packets
  • requirement: mandatory for IPv6 peers
  • use case: allows binding to specific source addresses in multi-homed environments
• Added Interface with:
  • behavior: selects which interface to use for this BFD peer
  • use case: binds the BFD session to a specific network interface

Related

• Plugin implementation: opnsense/plugins#5317