docs: Move explanation of hardened key syntax closer to KEY section

Before 7cedafc added the TREE
section, this line appeared right after the KEY section.

It doesn't really fit in its former location since it's the KEY
section that discusses derivation path syntax, not the TREE section.

Author: bitcoinhodler

doc/descriptors.md

@@ -90,12 +90,12 @@ Descriptors consist of several types of expressions. The top level expression is
     - Optionally followed by a single `/*` or `/*'` final step to denote all (direct) unhardened or hardened children.
     - The usage of hardened derivation steps requires providing the private key.
 
+(Anywhere a `'` suffix is permitted to denote hardened derivation, the suffix `h` can be used instead.)
+
 `TREE` expressions:
 - any `SCRIPT` expression
 - An open brace `{`, a `TREE` expression, a comma `,`, a `TREE` expression, and a closing brace `}`
 
-(Anywhere a `'` suffix is permitted to denote hardened derivation, the suffix `h` can be used instead.)
-
 `ADDR` expressions are any type of supported address:
 - P2PKH addresses (base58, of the form `1...` for mainnet or `[nm]...` for testnet). Note that P2PKH addresses in descriptors cannot be used for P2PK outputs (use the `pk` function instead).
 - P2SH addresses (base58, of the form `3...` for mainnet or `2...` for testnet, defined in [BIP 13](https://github.com/bitcoin/bips/blob/master/bip-0013.mediawiki)).