docs: Document [peers_in_max] and [peers_out_max] in the example config

[pete-csyn]

High Level Overview of Change

Documents [peers_in_max] and [peers_out_max] in cfg/xrpld-example.cfg. Comment
only — no behaviour change.

Context of Change

Both settings were added in #3616, which didn't touch a config file, so they have
never appeared in the example config. Today the only way to learn they exist — or
what they accept — is to read Config.cpp.

The behaviour most likely to cost someone time is that they must be set as a
pair. Configuring only one throws at startup:

Both sections [peers_in_max] and [peers_out_max] must be configured

which is easy to hit when trying to bound inbound connections alone. The valid
ranges also differ between the two (peers_in_max 0–1000, peers_out_max 10–1000),
and both are ignored when [peers_max] is present.

This documents the pair, the ranges, the both-or-neither requirement, and the
precedence, following the style of the surrounding stanzas. Verified against
src/xrpld/core/detail/Config.cpp on develop.

API Impact

• Public API: New feature (new methods and/or new fields)
• Public API: Breaking change (in general, breaking changes should only impact the next api_version)
• libxrpl change (any change that may affect libxrpl or dependents of libxrpl)
• Peer protocol change (must be backward compatible or bump the peer protocol version)

None — this changes only comments in the example config.