Update documentation to reflect the emoji -> symbol shift.

Closes jgm#112.

doc/cheatsheet.md

@@ -26,8 +26,6 @@ Leave a blank line between paragraphs.
 **Comments** `{% look like this`\
 `and can span multiple lines %}`.
 
-**Emojis**: `:+1:` → 👍, `:smiley:` → 😃 , etc.
-
 **Thematic break** (horizontal line): `***` or `---` on its own line.
 Can be longer than three characters, and may contain or be indented
 by spaces/tabs.

doc/syntax.html

@@ -259,7 +259,7 @@
 <li><a href="#footnote-reference" id="toc-footnote-reference">Footnote reference</a></li>
 <li><a href="#line-break" id="toc-line-break">Line break</a></li>
 <li><a href="#comment" id="toc-comment">Comment</a></li>
-<li><a href="#emoji" id="toc-emoji">Emoji</a></li>
+<li><a href="#symbols" id="toc-symbols">Symbols</a></li>
 <li><a href="#raw-inline" id="toc-raw-inline">Raw inline</a></li>
 <li><a href="#span" id="toc-span">Span</a></li>
 <li><a href="#inline-attributes" id="toc-inline-attributes">Inline attributes</a></li>
@@ -804,9 +804,12 @@ <h3>Comment</h3>
 </div>
 </div>
 </section>
-<section id="emoji" class="level3">
-<h3>Emoji</h3>
-<p>Emojis may be included by surrounding their aliases with <code>:</code> signs:</p>
+<section id="symbols" class="level3">
+<h3>Symbols</h3>
+<p>Surrounding a word with <code>:</code> signs creates a “symbol,” which by
+default is just rendered literally but may be treated specially
+by a filter. (For example, a filter could convert symbols to
+emojis. But this is not built into djot.)</p>
 <div class="example">
 <div class="djot">
 <pre><code>My reaction is :+1: :smiley:.</code></pre>

doc/syntax.md

@@ -285,9 +285,12 @@ attribute specifier that contains only a comment:
     Foo bar {% This is a comment, spanning
     multiple lines %} baz.
 
-### Emoji
+### Symbols
 
-Emojis may be included by surrounding their aliases with `:` signs:
+Surrounding a word with `:` signs creates a "symbol," which by
+default is just rendered literally but may be treated specially
+by a filter.  (For example, a filter could convert symbols to
+emojis. But this is not built into djot.)
 
     My reaction is :+1: :smiley:.