doc/guide: Reword CONFLICTS usage suggestions

Despite most CONFLICTS are automatically detected at package
installation time it is still a good idea to manually mark CONFLICTS
in order to fail as early as possible instead e.g. of failing at
package installation time after possibly installing several (maybe
not needed) dependencies.

Pointed out by discussion on pkgsrc-changes@ by <jperkin>:

 <https://mail-index.NetBSD.org/pkgsrc-changes/2018/09/09/msg180626.html>

Last two paragraphs from <jperkin>, thanks!


Index: fixes.xml
===================================================================
RCS file: /cvsroot/pkgsrc/doc/guide/files/fixes.xml,v
retrieving revision 1.145
diff -u -p -r1.145 fixes.xml
--- fixes.xml	6 Sep 2017 16:49:22 -0000	1.145
+++ fixes.xml	10 Sep 2018 11:45:47 -0000
@@ -427,31 +427,36 @@ DEPENDS+=       ImageMagick>=6.0:../../g
     installs the same set of files as another package in the pkgsrc
     tree or has the same <varname>PKGNAME</varname>.</para>
 
-    <para>These cases are handled automatically by the packaging tools
-    at package installation time and do not need to be handled
-    manually.</para>
-   
-    <para>In case the conflicts can not be recognized automatically
-    (e.g., packages using the same config file location but no other
-    shared files), you can set <varname>CONFLICTS</varname> to a
-    space-separated list of packages (including version string) your
-    package conflicts with.</para>
-
-    <para>For example, if both <filename role="pkg">foo/bar</filename>
-    and <filename role="pkg">foo/baz</filename>
-    use the same config file, you would set in
-    <filename>foo/bar/Makefile</filename>:</para>
-
-<programlisting>
-CONFLICTS=      baz-[0-9]*
-</programlisting>
-
-    <para>and in <filename>pkgsrc/foo/baz/Makefile</filename>:</para>
-
-<programlisting>
-CONFLICTS=      bar-[0-9]*
-</programlisting>
-
+    <para>For example, <filename role="pkg">x11/libXaw3d</filename>
+    and <filename role="pkg">x11/Xaw-Xpm</filename>
+    install the same shared library, thus you set in
+    <filename>pkgsrc/x11/libXaw3d/Makefile</filename>:</para>
+
+    <programlisting>
+CONFLICTS=      Xaw-Xpm-[0-9]*
+    </programlisting>
+
+    <para>and in <filename>pkgsrc/x11/Xaw-Xpm/Makefile</filename>:</para>
+
+    <programlisting>
+CONFLICTS=      libXaw3d-[0-9]*
+    </programlisting>
+
+    <para>&man.pkg.add.1 is able to detect attempts to install packages
+    that conflict with existing packages and abort. However, in many
+    situations this is too late in the process. Binary package managers
+    will not know about the conflict until they attempt to install the
+    package after already downloading it and all its dependencies.
+    Users may also waste time building a package and its dependencies
+    only to find out at the end that it conflicts with another package
+    they have installed.</para>
+ 
+    <para>To avoid these issues <varname>CONFLICTS</varname> entries
+    should be added in all cases where it is known that packages conflict
+    with each other.  These <varname>CONFLICTS</varname> entries are
+    exported in &man.pkg.summary.5 files and consumed by binary package
+    managers to inform users that packages cannot be installed onto
+    the target system.</para>
   </sect2>