gitweb/INSTALL: Simplify description of GITWEB_CONFIG_SYSTEM

The flow of the text describing GITWEB_CONFIG_SYSTEM and
GITWEB_CONFIG_COMMON in gitweb/INSTALL is awkward.  "This is bad. Oh
the other hand, better is broken. Therefore we do this." forces
readers to make multiple guesses while reading: "ok, bad, so you
plan to change it and warn us about upcoming change?  oh, not that,
changing it is bad, so we have to live with it?  oh, not that, there
is another one that is common and that is what we can use".

Better rewrite said paragraph to avoid such a mental roller-coaster in
the first place.

Signed-off-by: Junio Hamano <gitster@pobox.com>
Signed-off-by: Jakub Narebski <jnareb@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

gitweb/INSTALL

@@ -243,14 +243,11 @@ for gitweb (in gitweb/README), and gitweb.conf(5) manpage.
   GITWEB_CONFIG_SYSTEM build configuration variable, and override it
   through the GITWEB_CONFIG_SYSTEM environment variable.
 
-  Note that if per-instance configuration file exists, then system-wide
-  configuration is _not used at all_.  This is quite untypical and surprising
-  behavior.  On the other hand changing current behavior would break backwards
-  compatibility and can lead to unexpected changes in gitweb behavior.
-  Therefore gitweb also looks for common system-wide configuration file,
-  normally /etc/gitweb-common.conf (set during build time using build time
-  configuration variable GITWEB_CONFIG_COMMON, set it at runtime using
-  environment variable with the same name).  Settings from per-instance or
+  Note that the GITWEB_CONFIG_SYSTEM system-wide configuration file is
+  only used for instances that lack per-instance configuration file.
+  You can use GITWEB_CONFIG_COMMON common system-wide configuration
+  file (normally /etc/gitweb-common.conf) to keep common default
+  settings that apply to all instances.  Settings from per-instance or
   system-wide configuration file override those from common system-wide
   configuration file.