Add doc comments.

Author: mdaigle

src/Microsoft.Data.SqlClient/netcore/src/Microsoft/Data/SqlClient/SqlInternalConnectionTds.cs

@@ -958,7 +958,7 @@ protected override void InternalDeactivate()
             }
         }
 
-        [SuppressMessage("Microsoft.Globalization", "CA1303:DoNotPassLiteralsAsLocalizedParameters")] // copied from Triaged.cs
+        /// <inheritdoc/>
         internal override void ResetConnection()
         {
             // For implicit pooled connections, if connection reset behavior is specified,

src/Microsoft.Data.SqlClient/netfx/src/Microsoft/Data/SqlClient/SqlInternalConnectionTds.cs

@@ -966,7 +966,7 @@ protected override void InternalDeactivate()
             }
         }
 
-        [SuppressMessage("Microsoft.Globalization", "CA1303:DoNotPassLiteralsAsLocalizedParameters")] // copied from Triaged.cs
+        /// <inheritdoc/>
         internal override void ResetConnection()
         {
             // For implicit pooled connections, if connection reset behavior is specified,

src/Microsoft.Data.SqlClient/src/Microsoft/Data/ProviderBase/DbConnectionClosed.cs

@@ -33,6 +33,7 @@ internal override void CloseConnection(DbConnection owningObject, SqlConnectionF
             // not much to do here...
         }
 
+        /// <inheritdoc/>
         protected override void Deactivate() => ADP.ClosedConnectionError();
 
         public override void EnlistTransaction(System.Transactions.Transaction transaction) => throw ADP.ClosedConnectionError();
@@ -56,6 +57,7 @@ internal override bool TryOpenConnection(
             DbConnectionOptions userOptions) =>
             TryOpenConnectionInternal(outerConnection, connectionFactory, retry, userOptions);
 
+        /// <inheritdoc/>
         internal override void ResetConnection() => throw ADP.ClosedConnectionError();
     }
 

src/Microsoft.Data.SqlClient/src/Microsoft/Data/ProviderBase/DbConnectionInternal.cs

@@ -795,6 +795,11 @@ internal void PrePush(DbConnection expectedOwner)
         internal void RemoveWeakReference(object value) =>
             ReferenceCollection?.Remove(value);
 
+        /// <summary>
+        /// Idempotently resets the connection so that it may be recycled without leaking state.
+        /// May preserve transaction state if the connection is enlisted in a distributed transaction.
+        /// Should be called before the first action is taken on a recycled connection.
+        /// </summary>
         internal abstract void ResetConnection();
 
         internal void SetInStasis()
@@ -834,6 +839,11 @@ internal virtual bool TryReplaceConnection(
 
         #region Protected Methods
 
+        /// <summary>
+        /// Activates the connection, preparing it for active use.
+        /// An activated connection has an owner and is checked out from the connection pool (if pooling is enabled).
+        /// </summary>
+        /// <param name="transaction">The transaction in which the connection should enlist.</param>
         protected abstract void Activate(Transaction transaction);
 
         /// <summary>
@@ -850,6 +860,11 @@ protected virtual DbReferenceCollection CreateReferenceCollection()
             throw ADP.InternalError(ADP.InternalErrorCode.AttemptingToConstructReferenceCollectionOnStaticObject);
         }
 
+        /// <summary>
+        /// Deactivates the connection, cleaning up any state as necessary.
+        /// A deactivated connection is one that is no longer in active use and does not have an owner.
+        /// A deactivated connection may be open (connected to a server) and is checked into the connection pool (if pooling is enabled).
+        /// </summary>
         protected abstract void Deactivate();
 
         protected internal void DoNotPoolThisConnection()

src/Microsoft.Data.SqlClient/src/Microsoft/Data/SqlClient/SqlInternalConnection.cs

@@ -246,6 +246,7 @@ override protected DbReferenceCollection CreateReferenceCollection()
             return new SqlReferenceCollection();
         }
 
+        /// <inheritdoc/>
         override protected void Deactivate()
         {
             try