message: doc: Add developer documentation to isminetype
diff: class CWallet;
  class CScript;
  -/** IsMine() return codes */
  +/**
  + * IsMine() return codes, which depend on ScriptPubKeyMan implementation.
  + * Not every ScriptPubKeyMan covers all types, please refer to
  + * https://github.com/bitcoin/bitcoin/blob/master/doc/release-notes/release-notes-0.21.0.md#ismine-semantics
  + * for better understanding.
  + *
  + * For LegacyScriptPubKeyMan,
  + * ISMINE_NO: the scriptPubKey is not in the wallet;
  + * ISMINE_WATCH_ONLY: the scriptPubKey has been imported into the wallet;
  + * ISMINE_SPENDABLE: the scriptPubKey corresponds to an address owned by the wallet user (can spend with the private key);
  + * ISMINE_USED: the scriptPubKey corresponds to a used address owned by the wallet user;
  + * ISMINE_ALL: all ISMINE flags except for USED;
  + * ISMINE_ALL_USED: all ISMINE flags including USED;
  + * ISMINE_ENUM_ELEMENTS: the number of isminetype enum elements.
  + *
  + * For DescriptorScriptPubKeyMan and future ScriptPubKeyMan,
  + * ISMINE_NO: the scriptPubKey is not in the wallet;
  + * ISMINE_SPENDABLE: the scriptPubKey matches a scriptPubKey in the wallet.
  + * ISMINE_USED: the scriptPubKey corresponds to a used address owned by the wallet user.
  + *
  + */
  enum isminetype : unsigned int
  {
  ISMINE_NO = 0,
  