document new [markup] section

docs/_static/css/site.scss

@@ -350,6 +350,10 @@ main {
     margin-left: 1.2em;
   }
 
+  dl {
+    margin-bottom: 0.4em;
+  }
+
   .admonition {
 
     p + p {

docs/docs/configuration/server.rst

@@ -221,6 +221,37 @@ reply-to-self
 
     Do not forget to configure the client.
 
+Markup
+------
+
+Customize markup and sanitized HTML. Currently, only Markdown (via Misaka) is
+supported, but new languages are relatively easy to add.
+
+.. code-block:: ini
+
+    [markup]
+    options = strikethrough, superscript, autolink
+    allowed-elements =
+    allowed-attributes =
+
+options
+    `Misaka-specific Markdown extensions <http://misaka.61924.nl/api/>`_, all
+    flags starting with `EXT_` can be used there, separated by comma.
+
+allowed-elements
+    Additional HTML tags to allow in the generated output, comma-separated. By
+    default, only *a*, *blockquote*, *br*, *code*, *del*, *em*, *h1*, *h2*,
+    *h3*, *h4*, *h5*, *h6*, *hr*, *ins*, *li*, *ol*, *p*, *pre*, *strong*,
+    *table*, *tbody*, *td*, *th*, *thead* and *ul* are allowed.
+
+allowed-attributes
+    Additional HTML attributes (independent from elements) to allow in the
+    generated output, comma-separated. By default, only *align* and *href* are
+    allowed.
+
+To allow images in comments, you just need to add ``allowed-elements = img`` and
+``allowed-attributes = src``.
+
 
 Appendum
 --------

docs/isso.example.cfg

@@ -110,3 +110,22 @@ direct-reply = 3
 # comment. After the editing timeframe is gone, commenters can reply to their
 # own comments anyways.  Do not forget to configure the client.
 reply-to-self = false
+
+
+[markup]
+# Customize markup and sanitized HTML. Currently, only Markdown (via Misaka) is
+# supported, but new languages are relatively easy to add.
+
+# Misaka-specific Markdown extensions, all flags starting with EXT_ can be used
+# there, separated by comma.
+options = strikethrough, superscript, autolink
+
+# Additional HTML tags to allow in the generated output, comma-separated. By
+# default, only a, blockquote, br, code, del, em, h1, h2, h3, h4, h5, h6, hr,
+# ins, li, ol, p, pre, strong, table, tbody, td, th, thead and ul are allowed.
+allowed-elements =
+
+# Additional HTML attributes (independent from elements) to allow in the
+# generated output, comma-separated. By default, only align and href are
+# allowed.
+allowed-attributes =