Tweak wording in ERC20Snapshot docs

contracts/token/ERC20/ERC20Snapshot.sol

@@ -60,8 +60,8 @@ abstract contract ERC20Snapshot is ERC20 {
      *
      * Emits a {Snapshot} event that contains the same id.
      *
-     * {_snapshot} is `internal`: you must decide how to expose it externally. This can be done both by
-     * guarding it with a system such as {AccessControl}, or by leaving it open to the public. 
+     * {_snapshot} is `internal` and you have to decide how to expose it externally. It's usage may be restricted to a
+     * set of accounts, for example using {AccessControl}, or it may be open to the public.
      *
      * [WARNING]
      * ====