Assets: add documentation

Signed-off-by: Paul-Elliot <peada@free.fr>

Author: panglesd

doc/ocamldoc_differences.mld

@@ -33,6 +33,7 @@ The following describes the changes between what [odoc] understands and what’s
 - [odoc] has a better mechanism for disambiguating references in comments. See 'reference syntax' later in this document.
 - Built-in support for standalone [.mld] files. These are documents using the OCamldoc markup, but they’re rendered as distinct pages.
 - Structured output: [odoc] can produce output in a structured directory tree rather a set of files.
+- [odoc] support the inclusion of assets in the structured directory tree.
 - A few extra tags are supported:
   + [@returns] is a synonym for [@return]
   + [@raises] is a synonym for [@raise]
@@ -56,6 +57,7 @@ Additionally we support extra annotations:
 - [instance-variable] refers to instance variables
 - [label] refers to labels introduced in anchors
 - [page] refers to [.mld] pages as outlined above
+- [asset] refers assets as outlined above
 - [value] is recognised as [val]
 
 {3 Referencing items containing hyphens or dots}

doc/odoc_for_authors.mld

@@ -393,13 +393,14 @@ The prefixes supported are:
 - [instance-variable]
 - [section] (and the equivalent deprecated prefix [label]) - for referring to headings
 - [page] - for referring to [.mld] pages
+- [asset] - for referring to assets
 
 In some cases the element being referenced might have a hyphen, a dot or a space in the name,
 e.g. if trying to refer to a page from a [.mld] file "1.2.3.mld". In this case, the
 element name should be quoted with double quote marks:
 
 {v
-{!page-"1.2.3"}
+{!page-"1.2.3"}, {!asset-"file.txt"}
 v}