Clarify PaymentSplitter shares are static

(cherry picked from commit 4bf9ab4)

Author: frangio

contracts/finance/PaymentSplitter.sol

@@ -14,7 +14,8 @@ import "../utils/Context.sol";
  *
  * The split can be in equal parts or in any other arbitrary proportion. The way this is specified is by assigning each
  * account to a number of shares. Of all the Ether that this contract receives, each account will then be able to claim
- * an amount proportional to the percentage of total shares they were assigned.
+ * an amount proportional to the percentage of total shares they were assigned. The distribution of shares is set at the
+ * time of contract deployment and can't be updated thereafter.
  *
  * `PaymentSplitter` follows a _pull payment_ model. This means that payments are not automatically forwarded to the
  * accounts but kept in this contract, and the actual transfer is triggered as a separate step by calling the {release}