Update the docs and comments for decoding of prepared xacts.

 Update the docs and comments for decoding of prepared xacts.
 
 Commit a271a1b50e introduced decoding at prepare time in ReorderBuffer.
 This can lead to deadlock for out-of-core logical replication solutions
 that uses this feature to build distributed 2PC in case such transactions
 lock [user] catalog tables exclusively. They need to inform users to not
 have locks on catalog tables (via explicit LOCK command) in such
 transactions.
 
 Reported-by: Andres Freund
 Discussion: https://postgr.es/m/20210222222847.tpnb6eg3yiykzpky@alap3.anarazel.de