Mention bech32m character limit

BIP173 defines a 90 char limit, but we need 115. Most implementations
already remove this limit as lightning encodings are longer than 90 char
and use bech32, but still should be mentioned for wallet implementors.

bip-0000.mediawiki

@@ -168,6 +168,8 @@ A silent payment address is constructed in the following manner:
 *** The character "q", to represent a silent payment address of version 0
 *** The 64 byte concatenation of the receiver's public keys, ''B<sub>scan</sub> || B<sub>m</sub>''
 
+Note: [https://github.com/bitcoin/bips/blob/master/bip-0173.mediawiki] imposes a 90 character for bech32 strings, whereas a silent payment address requires 115 characters.
+
 === Outpoints hash ===
 
 The sender and receiver MUST calculate an outpoints hash for the transaction in the following manner: