Update ZipArchiveMode.Create doc comments

For context, see dotnet#49149. The aim is to reduce confusion around the fact that Create mode supports append (unlike `FileMode.Create`).

src/libraries/System.IO.Compression/src/System/IO/Compression/ZipArchiveMode.cs

@@ -13,10 +13,9 @@ public enum ZipArchiveMode
  /// </summary>
  Read,
  /// <summary>
- /// Only supports the creation of new archives.
- /// Only writing to newly created entries in the archive is permitted.
- /// Each entry in the archive can only be opened for writing once.
- /// If only one entry is written to at a time, data will be written to the underlying stream or file as soon as it is available.
+ /// Only supports appending new entries to a new or existing archive. Only writing to these newly-created entries is permitted.
+ /// Each such entry can only be opened for writing once, and only one entry may be open at a time.
+ /// Data written to entries will be directly written through to the underlying stream or file.
  /// The underlying stream must be writable, but need not be seekable.
  /// </summary>
  Create,