DOC: global directive server-state-base

Documentation relative to the new global directive server-state-file.
diff --git a/doc/configuration.txt b/doc/configuration.txt
index c4aa8f9..7508a28 100644
--- a/doc/configuration.txt
+++ b/doc/configuration.txt
@@ -561,6 +561,7 @@
    - nosplice
    - nogetaddrinfo
    - spread-checks
+   - server-state-base
    - tune.bufsize
    - tune.chksize
    - tune.comp.maxlevel
@@ -759,6 +760,10 @@
   word size (32 or 64). A better option consists in using the "process" setting
   of the "stats socket" line to force the process on each line.
 
+server-state-base <directory>
+  Specifies the directory prefix to be prepended in front of all servers state
+  file names which do not start with a '/'. Read below 'See also'.
+
 ssl-default-bind-ciphers <ciphers>
   This setting is only available when support for OpenSSL was built in. It sets
   the default string describing the list of cipher algorithms ("cipher suite")