Update documentation of gfsec-split's -c option

Update the manuals to reflect the new behavior of gfsec-split's
-c, --config option.

info/gfsecret.texi

@@ -185,9 +185,13 @@ directory part and file extension). For example, when called to split a
 file @file{/home/alice/Documents/mysecret.dat}, @command{gfsec-split} will
 generate a configuration file in
 @file{/home/alice/.config/gfsecret/mysecret.conf} (assuming the default
-value for the @var{XDG_CONFIG_HOME} environment variable). Use the
-option @option{-c} (@option{--config}) to write the configuration file
-elsewhere.
+value for the @var{XDG_CONFIG_HOME} environment variable).
+
+Use the option @option{-c} (@option{--config}) @var{name} to specify
+yourself the name of the configuration file. If @var{name} is a filename
+without extension and without a directory part, the configuration file
+will be written to @file{$XDG_CONFIG_HOME/gfsecret/@var{name}.conf}.
+Otherwise it will be written exactly at the specified location.
 
 @menu
 * Specifying Share URIs on the Command Line::
@@ -415,8 +419,7 @@ the available devices. She thus calls @command{gfsec-split} again as
 follows:
 
 @example
-$ gfsec-split \
-  -c /home/alice/.config/gfsecret/master.conf \
+$ gfsec-split -c master \
   /home/alice/.gnupg/private-keys-v1.d/47921A[...]3EA88A.key \
   file:///home/alice/.local/share/gfsecret/master-key \
   label://USBKEY/master-key \
@@ -424,7 +427,7 @@ $ gfsec-split \
 @end example
 
 Here, Alice explicitly sets the name of the configuration file to
-generate (with the @option{-c}). The default behavior of
+generate (with the @option{-c} option). The default behavior of
 @command{gfsec-split} would have created a configuration file named with
 the 40 characters of the keygrip, which would have been especially
 impractical.

man/gfsec-split.1.in

@@ -46,6 +46,10 @@ the secret with
 .BR gfsec-use (1)
 ) to the specified file. Default is the basename of
 the splitted file in $XDG_CONFIG_HOME/gfsecret.
+If \fiFILE\fR is a single filename without extension and
+without a directory part, the file will be placed under
+the $XDG_CONFIG_HOME/gfsecret directory with a .conf
+extension.
 .TP
 .BR -i ", " --interactive
 Present the user with an interactive menu to specify the