ISO-2022-JP encoder: document an oddity

At this point it does not seem worth it to require further implementation changes and risk compatibility issues, so instead document the quirk.

Closes #115.

encoding.bs

@@ -2650,6 +2650,15 @@ consumers of content generated with <a>GBK</a>'s <a for=/>encoder</a>.
 
 <h4 id=iso-2022-jp-encoder dfn export>ISO-2022-JP encoder</h4>
 
+<div class="note no-backref">
+ <p>The <a>ISO-2022-JP encoder</a> is the only <a for=/>encoder</a> for which the concetenation of
+ multiple outputs can result in an <a>error</a> when run through the <a>ISO-2022-JP decoder</a>.
+
+ <p class=example id=example-iso-2022-jp-encoder-oddity>Encoding U+00A5 gives 0x1B 0x28 0x4A 0x5C
+ 0x1B 0x28 0x42. Doing that twice, concatenating the results, and then decoding yields U+00A5 U+FFFD
+ U+00A5.
+</div>
+
 <p><a>ISO-2022-JP</a>'s <a for=/>encoder</a> has an associated
 <dfn>ISO-2022-JP encoder state</dfn> which is <dfn lt="ISO-2022-JP encoder ASCII">ASCII</dfn>,
 <dfn lt="ISO-2022-JP encoder Roman">Roman</dfn>, or