Update speech-synthesis-markup.md

Add several extra elements for Silence feature

Author: sally-baolian

articles/cognitive-services/Speech-Service/speech-synthesis-markup.md

@@ -390,7 +390,7 @@ Use the `mstts:silence` element to insert pauses before or after text, or betwee
 
 | Attribute | Description | Required or optional |
 | ---------- | ---------- | -------------------- |
-| `type` | Specifies the location of silence to be added: <ul><li>`Leading` – At the beginning of text </li><li>`Tailing` – At the end of text </li><li>`Sentenceboundary` – Between adjacent sentences </li></ul> | Required  |
+| `type` | Specifies the location of silence to be added: <ul><li>`Leading` – It indicates the beginning of the text, but has a minimum value of 50 ms. That is, if you set a value of less than 50 ms for this type, then the silence length will actually be 50 ms. </li><li>`Leading-exact` – It indicates the beginning of the text, and the value set for this type is an absolute silence length.</li><li>`Tailing` – It indicates the end of text, and the actual silence length for this type includes the silence after the last word and the vaule you set for this type.</li><li>`Tailing-exact` – It indicates the end of the text, and the value set for this type is an absolute silence length.</li><li>`Sentenceboundary` – It indicates between adjacent sentences, and the actual silence length for this type includes the silence after the last word in the previous sentence, the vaule you set for this type, and the silence before the starting word in the next sentence. </li><li>`Sentenceboundary-exact` – It indicates between adjacent sentences, and the value set for this type is an absolute silence length. </li></ul> <ul>Each type with the suffix "-exact" takes precedence over its corresponding type. For example, if you set both `Leading` and `Leading-exact` types, the `Leading-exact` type will take effect.</ul>| Required  |
 | `Value`   | Specifies the absolute duration of a pause in seconds or milliseconds. This value should be set less than 5,000 ms. Examples of valid values are `2s` and `500ms`.| Required |
 
 **Example**