Document thread safety (Python-Markdown#882)

Fixed Python-Markdown#812.

.spell-dict

@@ -126,6 +126,7 @@ Treeprocessor
 Treeprocessors
 tuple
 tuples
+unescape
 unordered
 untrusted
 UTF

docs/change_log/release-3.2.md

@@ -53,14 +53,15 @@ continue to see the old behavior.
 
 The following new features have been included in the release:
 
+* Document thread safety (#812).
 * Markdown parsing in HTML has been exposed via a separate extension called
   [`md_in_html`](../extensions/md_in_html.md).
-
 * Add support for Python 3.8.
 
 ## Bug fixes
 
 The following bug fixes are included in the 3.2 release:
 
+* Unescape backslash-escaped characters in TOC ids (#864).
 * Refactor bold and italic logic in order to solve complex nesting issues (#792).
 * Always wrap CodeHilite code in <code> tags (#862).

docs/reference.md

@@ -225,7 +225,13 @@ __encoding__{: #encoding }
 The same options are available when initializing the `markdown.Markdown` class
 as on the [`markdown.markdown`](#markdown) function, except that the class does
 **not** accept a source text string on initialization. Rather, the source text
-string must be passed to one of two instance methods:
+string must be passed to one of two instance methods.
+
+!!! warning
+
+    Instances of the `markdown.Markdown` class are only thread safe within
+    the thread they were created in. A single instance should not be accessed
+    from multiple threads.
 
 #### Markdown.convert(source) {: #convert }