docs: Reformat and update the text markup documentation.

docs/index.md

   - [MBROLA Voices](mbrola.md)
   - [Phoneme Tables](phontab.md)
 - [Intonation](intonation.md)
-- [Text Markup](markup.md)
+- [SSML and HTML Support](markup.md)
 - [License](../COPYING)
 
 ----------

docs/languages/gmw/en.md

    Creative Commons Attribution-Sharealike 3.0 Unported License (CC-BY-SA).
 
 2. <a name="ref2"></a> Wikipedia.
-   [IPA chart for English dialects](https://en.wikipedia.org/wiki/International_Phonetic_Alphabet_chart_for_English_dialects).
-   2018. Creative Commons Attribution-Sharealike 3.0 Unported License (CC-BY-SA).
+   [IPA chart for English dialects](https://en.wikipedia.org/wiki/International_Phonetic_Alphabet_chart_for_English_dialects). 2018.
+   Creative Commons Attribution-Sharealike 3.0 Unported License (CC-BY-SA).

docs/markup.md

-# Text Markup
-
-- [SSML: Speech Synthesis Markup Language](#ssml-speech-synthesis-markup-language)
+# SSML and HTML Support
+
+- [SSML (Speech Synthesis Markup Language)](#ssml-speech-synthesis-markup-language)
+  - [speak](#speak)
+  - [voice](#voice)
+  - [prosody](#prosody)
+  - [say-as](#say-as)
+  - [mark](#mark)
+  - [s](#s)
+  - [p](#p)
+  - [sub](#sub)
+  - [tts:style](#tts-style)
+  - [audio](#audio)
+  - [emphasis](#emphasis)
+  - [break](#break)
 - [HTML](#html)
+- [References](#references)
+  - [SSML](#ssml)
+  - [HTML](#html-1)
 
 -----
 
-## SSML: Speech Synthesis Markup Language
+## SSML (Speech Synthesis Markup Language)
 
 The following markup tags and attributes are recognised:
 
-**\<speak\>**
+### speak
 
 *   xml:base   (the value is just passed back as a parameter with the UriCallback() function)
 *   xml:lang
 
-**\<voice\>**
+### voice
 
 *   xml:lang
 *   name
 *   variant
 *   gender
 
-**\<prosody\>**
+### prosody
 
 *   rate
 *   volume
 *   pitch
 *   range
 
-**\<say-as\>**
+### say-as
 
 *   interpret-as="characters"
 *   interpret-as="characters"  format="glyphs"
 *   interpret-as="tts:char"
 *   interpret-as="tts:digits"
 
-**\<mark\>** name
+### mark
+
+*   name
 
-**\<s\>**
+### s
 
 *   xml:lang
 
-**\<p\>**
+### p
 
 *   xml:lang
 
-**\<sub\>** alias
+### sub
+
+*   alias
 
-**\<tts:style\>**
+### tts:style
 
 *   field="punctuation"   mode=none,all,some
 *   field="capital\_letters"   mode=no,spelling,icon,pitch
 
-**\<audio\>** src
+### audio
 
-**\<emphasis\>**
+*   src
+
+### emphasis
 
 *   level
 
-**\<break\>**
+### break
 
 *   strength
 *   time
 eSpeak can speak HTML text directly, or text containing both SSML and HTML markup.  
 Any unrecognised tags are ignored.
 
-The following tags case a sentence break.  
-**\<br\>  \<dd\>   \<li\>   \<img\>   \<td\>**
+The following tags cause a sentence break:
+
+*   `br`
+*   `dd`
+*   `li`
+*   `img`
+*   `td`
+
+The following tags cause a paragraph break:
+
+*   `h1`
+*   `h2`
+*   `h3`
+*   `h4`
+*   `hr`
+
+Text between the following tags is ignored:
+
+*   `script`
+*   `style`
+
+## References
+
+### SSML
+
+1. [Speech Synthesis Markup Language (SSML) Version 1.0](https://www.w3.org/TR/2009/REC-speech-synthesis-20090303/).
+   W3C Recommendation, 3 March 2009. W3C.
+1. [Speech Synthesis Markup Language (SSML) Version 1.1](http://www.w3.org/TR/2010/REC-speech-synthesis11-20100907/).
+   W3C Recommendation, 7 September 2010. W3C.
+1. [SSML 1.0 say-as attribute values](http://www.w3.org/TR/2005/NOTE-ssml-sayas-20050526).
+   W3C NOTE, 26 May 2005. W3C.
 
-The following tags case a paragraph break.  
-**\<h1\>   \<h2\>   \<h3\>   \<h4\>   \<hr\>**
+### HTML
 
-Text between the following tags is ignored.  
-**\<script\>   ...   \</script\>**  
-**\<style\>   ...   \</style\>**
+1.  [HTML 5.2](https://www.w3.org/TR/2017/REC-html52-20171214/).
+    W3C Recommendation, 14 December 2017. W3C.
+1.  [HTML Living Standard](https://html.spec.whatwg.org/multipage/).
+    Continually updated. WHATWG.