git-commit.txt: better description what it does

The description of git-commit jumps right into the commit content, which
is important, but it fails to mention how the commit is "added" to the
repository. Update the first paragraph saying a bit more about branch
update to fill this gap.

While at there, add a couple linkgit references when the command is
first mentioned.

Helped-by: Eric Sunshine <sunshine@sunshineco.com>
Signed-off-by: Nguyễn Thái Ngọc Duy <pclouds@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/git-commit.txt

@@ -17,16 +17,20 @@ SYNOPSIS
 
 DESCRIPTION
 -----------
-Stores the current contents of the index in a new commit along
+Create a new commit containing the current contents of the index and
-with a log message from the user describing the changes.
+the given log message describing the changes. The new commit is a
+direct child of HEAD, usually the tip of the current branch, and the
+branch is updated to point to it (unless no branch is associated with
+the working tree, in which case HEAD is "detached" as described in
+linkgit:git-checkout[1]).
 
-The content to be added can be specified in several ways:
+The content to be committed can be specified in several ways:
 
-1. by using 'git add' to incrementally "add" changes to the
+1. by using linkgit:git-add[1] to incrementally "add" changes to the
-   index before using the 'commit' command (Note: even modified
+   index before using the 'commit' command (Note: even modified files
-   files must be "added");
+   must be "added");
 
-2. by using 'git rm' to remove files from the working tree
+2. by using linkgit:git-rm[1] to remove files from the working tree
    and the index, again before using the 'commit' command;
 
 3. by listing files as arguments to the 'commit' command