init: document init.defaultBranch better

Our documentation does not mention any future plan to change 'master' to
other value. It is a good idea to document this, though.

Initial-patch-by: Junio C Hamano <gitster@pobox.com>
Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Author: dscho

Documentation/git-init.txt

@@ -20,8 +20,9 @@ DESCRIPTION
 
 This command creates an empty Git repository - basically a `.git`
 directory with subdirectories for `objects`, `refs/heads`,
-`refs/tags`, and template files.  An initial `HEAD` file that
-references the HEAD of the master branch is also created.
+`refs/tags`, and template files.  An initial branch without any
+commits will be created (see the `--initial-branch` option below
+for its name).
 
 If the `$GIT_DIR` environment variable is set then it specifies a path
 to use instead of `./.git` for the base of the repository.
@@ -73,8 +74,10 @@ If this is reinitialization, the repository will be moved to the specified path.
 -b <branch-name>::
 --initial-branch=<branch-name>::
 
-Use the specified name for the initial branch in the newly created repository.
-If not specified, fall back to the default name: `master`.
+Use the specified name for the initial branch in the newly created
+repository.  If not specified, fall back to the default name (currently
+`master`, but this is subject to change in the future; the name can be
+customized via the `init.defaultBranch` configuration variable).
 
 --shared[=(false|true|umask|group|all|world|everybody|0xxx)]::