Add more explanatory text to the C-LINK section

The new text explains how to add regular markdown links and how to add
link targets. In particular it makes it clear that rustdoc link
targets go at the end of the docstring rather than inline.

Author: nicholasbishop

src/documentation.md

@@ -146,19 +146,25 @@ The unsafe [`std::ptr::read`] requires the following of the caller.
 <a id="c-link"></a>
 ## Prose contains hyperlinks to relevant things (C-LINK)
 
-Links to methods within the same type usually look like this:
+Regular links can be added inline with the usual markdown syntax of
+`[text](url)`. Links to other types can be added by marking them with
+``[`text`]``, then adding the link target in a new line at the end of
+the docstring with ``[`text`]: <target>``, where `<target>` is
+described below.
+
+Link targets to methods within the same type usually look like this:
 
 ```md
 [`serialize_struct`]: #method.serialize_struct
 ```
 
-Links to other types usually look like this:
+Link targets to other types usually look like this:
 
 ```md
 [`Deserialize`]: trait.Deserialize.html
 ```
 
-Links may also point to a parent or child module:
+Link targets may also point to a parent or child module:
 
 ```md
 [`Value`]: ../enum.Value.html