[doc] Rework docs

* Minor formatting such as adjusting links
* Move sections of `doc/multiwallet-qt.md` to the source code and delete
  the file, as it is outdated
* Fix typo in the release notes
* Amend release process to mention update of BLOCK_CHAIN_SIZE

src/qt/bitcoingui.cpp

@@ -74,6 +74,8 @@ const std::string BitcoinGUI::DEFAULT_UIPLATFORM =
 #endif
         ;
 
+/** Display name for default wallet name. Uses tilde to avoid name
+ * collisions in the future with additional wallets */
 const QString BitcoinGUI::DEFAULT_WALLET = "~Default";
 
 BitcoinGUI::BitcoinGUI(const PlatformStyle *_platformStyle, const NetworkStyle *networkStyle, QWidget *parent) :

src/qt/walletframe.h

@@ -19,6 +19,13 @@ QT_BEGIN_NAMESPACE
 class QStackedWidget;
 QT_END_NAMESPACE
 
+/**
+ * A container for embedding all wallet-related
+ * controls into BitcoinGUI. The purpose of this class is to allow future
+ * refinements of the wallet controls with minimal need for further
+ * modifications to BitcoinGUI, thus greatly simplifying merges while
+ * reducing the risk of breaking top-level stuff.
+ */
 class WalletFrame : public QFrame
 {
     Q_OBJECT