[jhbuild] Document the JHBUILD_RUN_AS_ROOT envvar

[Emmanuele Bassi <ebassi src gnome org>]

commit a212218a315d408f6f0ecea008c142fd89a8b6ba
Author: Emmanuele Bassi <ebassi gnome org>
Date:   Fri Jun 23 16:39:11 2017 +0100

    Document the JHBUILD_RUN_AS_ROOT envvar
    
    https://bugzilla.gnome.org/show_bug.cgi?id=783965

 doc/C/index.docbook |   17 +++++++++++++++++
 1 files changed, 17 insertions(+), 0 deletions(-)
---
diff --git a/doc/C/index.docbook b/doc/C/index.docbook
index 8c21f9d..36cd2da 100644
--- a/doc/C/index.docbook
+++ b/doc/C/index.docbook
@@ -709,6 +709,23 @@ jhbuildbot_password = 'password'
       </varlistentry>
     </variablelist>
 
+    <para>Additionally, JHBuild checks the presence, and value, of the
+      following environment variables:</para>
+
+    <variablelist>
+      <varlistentry>
+        <term><envar>JHBUILD_RUN_AS_ROOT</envar></term>
+        <listitem>
+          <simpara>By default, JHBuild does not run when using the root user,
+            to avoid destructive operations that may affect the whole system.
+            In certain cases, though, it's acceptable to run JHBuild as root;
+            for instance, inside a container, or in a continuous integration
+            environment. Exporting the <envar>JHBUILD_RUN_AS_ROOT</envar> will
+            disable the user id check at startup.</simpara>
+        </listitem>
+      </varlistentry>
+    </variablelist>
+
     <para>Command specific options are listed below.</para>
 
     <section id="command-reference-autobuild">