Clarify tlides and backticks in info strings.

We disallow backticks in info strings after backtick fences
only. Both backticks and tildes are allowed in info strings
after tilde fences.  Added example.

Closes #119.

spec.txt

@@ -1584,8 +1584,8 @@ begins with a code fence, indented no more than three spaces.
 
 The line with the opening code fence may optionally contain some text
 following the code fence; this is trimmed of leading and trailing
-whitespace and called the [info string](@).
-The [info string] may not contain any backtick
+whitespace and called the [info string](@). If the [info string] comes
+after a backtick fence, it may not contain any backtick
 characters.  (The reason for this restriction is that otherwise
 some inline code would be incorrectly interpreted as the
 beginning of a fenced code block.)
@@ -1973,6 +1973,18 @@ foo</p>
 ````````````````````````````````
 
 
+[Info strings] for tilde code blocks can contain backticks and tildes:
+
+```````````````````````````````` example
+~~~ aa ``` ~~~
+foo
+~~~
+.
+<pre><code class="language-aa">foo
+</code></pre>
+````````````````````````````````
+
+
 Closing code fences cannot have [info strings]:
 
 ```````````````````````````````` example