[jhbuild] doc: Document shallow_clone option

[Christoph Reiter <creiter src gnome org>]

commit eb3461564067e88b94b97d9ea0d0cb7d0083e946
Author: Diego Escalante Urrelo <diegoe gnome org>
Date:   Sat Aug 1 17:18:01 2020 -0500

    doc: Document shallow_clone option

 doc/C/index.docbook | 13 +++++++++++++
 1 file changed, 13 insertions(+)
---
diff --git a/doc/C/index.docbook b/doc/C/index.docbook
index 0a8e9ee25..20fb5c47b 100644
--- a/doc/C/index.docbook
+++ b/doc/C/index.docbook
@@ -2294,6 +2294,19 @@ Optional packages: (JHBuild will build the missing packages)
 <programlisting>repos['git.gnome.org'] = 'ssh://username git gnome org/git/'</programlisting>
           </listitem>
         </varlistentry>
+        <varlistentry id="cfg-shallow-clone">
+          <term>
+            <varname>shallow_clone</varname>
+          </term>
+          <listitem>
+              <simpara>A boolean value specifying if JHBuild should prefer
+                smaller version control downloads. Equivalent to
+                <command>git clone --depth 1</command> or
+                <command>bzr co --light</command>.
+                Defaults to <constant>False</constant>.
+             </simpara>
+          </listitem>
+        </varlistentry>
         <varlistentry id="cfg-skip">
           <term>
             <varname>skip</varname>