Reasons for rename:
- Default value is
<datadir>/walletsso calling it-walletsdirinstead of-walletdirwould be more internally consistent. - Directory can contain more than one wallet so plural makes sense
- This makes it harder to confuse
-walletdiroption with-walletoption if we store wallets in their own directories as proposed in #11466 (comment) and implemented in https://github.com/bitcoin/bitcoin/pull/12216