Rephrase comments to make them clearer

The use of "we" when referring to the active backend might be
misunderstood, so rephrase to make it clearer who is performing
the actions discussed in the comment.

Author: Junwang Zhao <zhjwpku@gmail.com>
Reviewed-by: Erikjan Rijkers <er@xs4all.nl>
Reviewed-by: Robert Treat <rob@xzilla.net>
Reviewed-by: Tom Lane <tgl@sss.pgh.pa.us>
Discussion: https://postgr.es/m/CAEG8a3LRSMqkvjiURiJoSi4aGWORpiXUmUfQQK5PaD6WfPzu3w@mail.gmail.com

src/backend/access/transam/xact.c

@@ -3140,7 +3140,7 @@ CommitTransactionCommand(void)
 			break;
 
 			/*
-			 * We were just issued a SAVEPOINT inside a transaction block.
+			 * The user issued a SAVEPOINT inside a transaction block.
 			 * Start a subtransaction.  (DefineSavepoint already did
 			 * PushTransaction, so as to have someplace to put the SUBBEGIN
 			 * state.)
@@ -3151,7 +3151,7 @@ CommitTransactionCommand(void)
 			break;
 
 			/*
-			 * We were issued a RELEASE command, so we end the current
+			 * The user issued a RELEASE command, so we end the current
 			 * subtransaction and return to the parent transaction. The parent
 			 * might be ended too, so repeat till we find an INPROGRESS
 			 * transaction or subtransaction.
@@ -3168,7 +3168,7 @@ CommitTransactionCommand(void)
 			break;
 
 			/*
-			 * We were issued a COMMIT, so we end the current subtransaction
+			 * The user issued a COMMIT, so we end the current subtransaction
 			 * hierarchy and perform final commit. We do this by rolling up
 			 * any subtransactions into their parent, which leads to O(N^2)
 			 * operations with respect to resource owners - this isn't that