waddrmgr: Update more documentation for walletdb.
waddrmgr: Update more documentation for walletdb.

synrg-labs/btcwallet,jrick/dcrwallet,Roasbeef/btcwallet,synrg-labs/btcwallet,Stash-Crypto/btcwallet,jrick/btcwallet,jrick/btcwallet,jrick/dcrwallet,btcsuite/btcwallet,decred/dcrwallet,decred/dcrwallet,Stash-Crypto/btcwallet,btcsuite/btcwallet,Roasbeef/btcwallet