document _requireZeroValue

contracts/proxy/transparent/TransparentUpgradeableProxy.sol

@@ -56,7 +56,7 @@ contract TransparentUpgradeableProxy is ERC1967Proxy {
      * `0xb53127684a568b3173ae13b9f8a6016e243e63b6e8ee1178d6a717850b5d6103`
      */
     function admin() external payable ifAdmin returns (address admin_) {
-        _nonPayable();
+        _requireZeroValue();
         admin_ = _getAdmin();
     }
 
@@ -70,7 +70,7 @@ contract TransparentUpgradeableProxy is ERC1967Proxy {
      * `0x360894a13ba1a3210667c828492db98dca3e2076cc3735a920a3ca505d382bbc`
      */
     function implementation() external payable ifAdmin returns (address implementation_) {
-        _nonPayable();
+        _requireZeroValue();
         implementation_ = _implementation();
     }
 
@@ -82,7 +82,7 @@ contract TransparentUpgradeableProxy is ERC1967Proxy {
      * NOTE: Only the admin can call this function. See {ProxyAdmin-changeProxyAdmin}.
      */
     function changeAdmin(address newAdmin) external payable virtual ifAdmin {
-        _nonPayable();
+        _requireZeroValue();
         _changeAdmin(newAdmin);
     }
 
@@ -92,7 +92,7 @@ contract TransparentUpgradeableProxy is ERC1967Proxy {
      * NOTE: Only the admin can call this function. See {ProxyAdmin-upgrade}.
      */
     function upgradeTo(address newImplementation) external payable ifAdmin {
-        _nonPayable();
+        _requireZeroValue();
         _upgradeToAndCall(newImplementation, bytes(""), false);
     }
 
@@ -122,7 +122,11 @@ contract TransparentUpgradeableProxy is ERC1967Proxy {
         super._beforeFallback();
     }
 
-    function _nonPayable() private {
+    /**
+     * @dev To keep this contract fully transparent, all `ifAdmin` functions must be payable. This helper is here to
+     * emulate some proxy functions being non-payable while still allowing value to pass through.
+     */
+    function _requireZeroValue() private {
         require(msg.value == 0);
     }
 }