docs: Correct config documentation to match code

[xrpl365]

Summary

Several documented defaults in cfg/xrpld-example.cfg no longer match the
values the code actually uses, and src/xrpld/app/rdb/README.md documents a
config section name ([relational_db]) that the code no longer reads (it reads
[sqdb]). This PR corrects the documentation to match the code. No behavior
change: comments and docs only.

Changes

cfg/xrpld-example.cfg (comment-only):

• compress_level: documented default corrected from 3 to 8.
• transaction_queue.minimum_txn_in_ledger: 5 to 32.
• transaction_queue.target_txn_in_ledger: 50 to 256.
• transaction_queue.minimum_escalation_multiplier: 500 to 128000
  (the value is expressed in fee levels: base fee level 256 times 500).
• node_db cache_size / cache_age: the documented fixed 16384 / 5 min
  are stale; when unset, the effective values are derived from [node_size].

src/xrpld/app/rdb/README.md:

• Correct the documented config section from [relational_db] to [sqdb].

How to verify

Each corrected value can be checked against the source:

Documented value	Old doc	Corrected to	Source of truth
compress_level default	3	8	src/libxrpl/server/Port.cpp:309 (valueOr(Keys::kCompressLevel, 8))
minimum_txn_in_ledger	5	32	src/xrpld/app/misc/TxQ.h:111 (minimumTxnInLedger = 32)
target_txn_in_ledger	50	256	src/xrpld/app/misc/TxQ.h:121 (targetTxnInLedger = 256)
minimum_escalation_multiplier	500	128000	src/xrpld/app/misc/TxQ.h:106 (minimumEscalationMultiplier = kBaseLevel * 500, kBaseLevel = 256)
cache_size / cache_age	16384 / 5 min	node-size derived	src/xrpld/app/misc/SHAMapStoreImp.cpp:198-205 (uses SizedItem::TreeCacheSize / TreeCacheAge when unset)
README section name	[relational_db]	[sqdb]	src/libxrpl/rdb/SociDB.cpp:55 (config.section(Sections::kSqdb))

Type of change

docs: documentation only, no functional change.