doc: update unstable version naming conventions

The previous explanation did not make it clear where the word
"unstable" should go when name is split into pname and version.

By putting it at the end, we can get builtins.parseDrvName to parse
things nicely:

> builtins.parseDrvName "pname-2020-10-17-unstable"

{ name = "pname"; version = "2020-10-17-unstable"; }

It is not good to put it in the pname, because then the pname will not
match with the attrpath.

doc/contributing/coding-conventions.xml

@@ -229,7 +229,7 @@ args.stdenv.mkDerivation (args // {
     </listitem>
     <listitem>
      <para>
-      If a package is not a release but a commit from a repository, then the version part of the name <emphasis>must</emphasis> be the date of that (fetched) commit. The date <emphasis>must</emphasis> be in <literal>"YYYY-MM-DD"</literal> format. Also append <literal>"unstable"</literal> to the name - e.g., <literal>"pkgname-unstable-2014-09-23"</literal>.
+      If a package is not a release but a commit from a repository, then the version part of the name <emphasis>must</emphasis> be the commit date of that (fetched) commit in <literal>"YYYY-MM-DD"</literal> format, followed by <literal>"-unstable"</literal>. For example, <literal>"pkgname-2014-09-23-unstable"</literal>.
      </para>
     </listitem>
     <listitem>