Add NatSpecs to Initializable (#11252)

packages/protocol/contracts/common/Initializable.sol

@@ -1,15 +1,30 @@
 // SPDX-License-Identifier: LGPL-3.0-only
 pragma solidity >=0.5.13 <0.9.0;
 
+/**
+ * @title Used with proxied contracts that have an `initialize` function.
+ * @notice Ensures the `initialize` function:
+ *         - gets called only once
+ *         - cannot be called on the logic contract.
+ */
 contract Initializable {
   bool public initialized;
 
+  /**
+   * @notice Ensures the initializer function cannot be called more than once.
+   */
   modifier initializer() {
     require(!initialized, "contract already initialized");
     initialized = true;
     _;
   }
 
+  /**
+   * @notice By default, ensures that the `initialize` function cannot be called
+   * on the logic contract.
+   * @param testingDeployment When set to true, allows the `initialize` function
+   * to be called, which is useful in testing when not setting up with a Proxy.
+   */
   constructor(bool testingDeployment) public {
     if (!testingDeployment) {
       initialized = true;