Add another option to pip3 install

Add a convenience option for &root;
Add --no-cache-dir to six and asciidoc.
Explain --no-cache-dir in pip3-cmd-explain.

general.ent

@@ -103,6 +103,10 @@
 <!ENTITY elinks-url           "http://elinks.or.cz">
 <!ENTITY w3m-url              "http://w3m.sourceforge.net/">
 
+<!-- Convenience entities -->
+<!ENTITY root                 "<systemitem class='username'>root</systemitem>">
+
+
 <!-- These entities are used to identify versions of LFS
      packages referenced throughout the BLFS book. -->
 <!ENTITY lfs-autoconf-version         "2.69">

general/prog/asciidoc.xml

@@ -104,7 +104,7 @@
       Now, as the <systemitem class="username">root</systemitem> user:
     </para>
 
-<screen role="root"><userinput>pip3 install --no-index --find-links dist asciidoc</userinput></screen>
+<screen role="root"><userinput>pip3 install --no-index --find-links dist --no-cache-dir asciidoc</userinput></screen>
 
     </sect3>
 

general/prog/six.xml

@@ -114,7 +114,7 @@ python3 setup.py build</userinput></screen>
 python3 setup.py install - -optimize=1</userinput></screen>
 -->
 
-<screen role="root"><userinput>pip3 install --no-index --find-links dist six</userinput></screen>
+<screen role="root"><userinput>pip3 install --no-index --find-links dist --no-cache-dir six</userinput></screen>
 
     </sect3>
 

xincludes/pip3-cmd-explain.xml

@@ -1,6 +1,9 @@
 <?xml version="1.0" encoding="ISO-8859-1"?>
 <!DOCTYPE note PUBLIC "-//OASIS//DTD DocBook XML V4.5//EN"
-  "http://www.oasis-open.org/docbook/xml/4.5/docbookx.dtd" >
+  "http://www.oasis-open.org/docbook/xml/4.5/docbookx.dtd" [
+  <!ENTITY % general-entities SYSTEM "../general.ent">
+  %general-entities;
+  ]>
 
   <sect3 role="commands">
     <title>Command Explanations</title>
@@ -33,5 +36,10 @@
       <filename class="directory">dist</filename>.
     </para>
 
+    <para>
+      <option>--no-cache-dir</option>: diables the cache to prevent
+      a warning when installing as the &root; user.
+    </para>
+
   </sect3>