use XMLLING_INDENT environment variable to control indentation for the xmllint "--format" option
diff --git a/doc/xmllint.xml b/doc/xmllint.xml
index 9bfbf64..356e456 100644
--- a/doc/xmllint.xml
+++ b/doc/xmllint.xml
@@ -330,7 +330,9 @@
<term><option>--format</option></term>
<listitem>
<simpara>
- Reformat and reindent the output.
+ Reformat and reindent the output. The $XMLLINT_INDENT
+ environment variable controls the indentation (default
+ value is two spaces " ").
</simpara>
</listitem>
</varlistentry>