changing doc format and HTML literals

site/doc/pages/substr.txt

@@ -8,7 +8,10 @@ start_pos : zero-based start position; negative values count from the end of the
 length : optional substring length; negative values trim bytes from the end of the result range
 return value : extracted substring, or an empty string if the requested range is outside the source string
 
-:desc
+:see
+>string
+
+:content
 Extracts part of a string using PHP-style start and length semantics.
 
 Use the two-argument form to return everything from `start_pos` to the end of the string.
@@ -17,9 +20,7 @@ If `start_pos` is negative, counting starts from the end of the string.
 
 If `length` is negative, the returned range stops that many bytes before the end of the string.
 
-:see
->string
+Related:
 
-:related
-**PHP:** `substr()`
-**JavaScript / Node.js:** `String.prototype.slice()` or `substring()`
+- PHP: `substr()`
+- JavaScript / Node.js: `String.prototype.slice()` or `substring()`