Issue #10546: UTF-16-LE and UTF-16-BE *do* support non-BMP characters

Fix the doc and add tests.
diff --git a/Doc/library/codecs.rst b/Doc/library/codecs.rst
index 5416d3b..26e31a4 100644
--- a/Doc/library/codecs.rst
+++ b/Doc/library/codecs.rst
@@ -1114,9 +1114,9 @@
 +-----------------+--------------------------------+--------------------------------+
 | utf_16          | U16, utf16                     | all languages                  |
 +-----------------+--------------------------------+--------------------------------+
-| utf_16_be       | UTF-16BE                       | all languages (BMP only)       |
+| utf_16_be       | UTF-16BE                       | all languages                  |
 +-----------------+--------------------------------+--------------------------------+
-| utf_16_le       | UTF-16LE                       | all languages (BMP only)       |
+| utf_16_le       | UTF-16LE                       | all languages                  |
 +-----------------+--------------------------------+--------------------------------+
 | utf_7           | U7, unicode-1-1-utf-7          | all languages                  |
 +-----------------+--------------------------------+--------------------------------+