doc/git-config: explain --file instead of referring to GIT_CONFIG

The explanation for the --file option only refers to GIT_CONFIG. This
redirection to an environment variable is confusing, but doubly so
because the description of GIT_CONFIG is out of date.

Let's describe --file from scratch, detailing both the reading and
writing behavior as we do for other similar options like --system, etc.

Signed-off-by: Jeff King <[email protected]>
Reviewed-by: Taylor Blau <[email protected]>
Signed-off-by: Junio C Hamano <[email protected]>

Author: peff

Documentation/git-config.txt

@@ -143,7 +143,13 @@ See also <<FILES>>.
 
 -f config-file::
 --file config-file::
-	Use the given config file instead of the one specified by GIT_CONFIG.
+	For writing options: write to the specified file rather than the
+	repository `.git/config`.
++
+For reading options: read only from the specified file rather than from all
+available files.
++
+See also <<FILES>>.
 
 --blob blob::
 	Similar to `--file` but use the given blob instead of a file. E.g.