git-tag.txt: list all modes in the description

Currently, the description sounds as if it applied always, but most of
its content is true in "create tag mode" only.

Make this clearer by listing all modes upfront.

Also, sneak in some linguistic improvements and make it clearer that
lightweight tags are "created" because "written" may be misread as
"are output".

Signed-off-by: Michael J Gruber <git@drmicha.warpmail.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/git-tag.txt

@@ -18,21 +18,22 @@ SYNOPSIS
 DESCRIPTION
 -----------
 
-Adds a tag reference in `.git/refs/tags/`.
+Add a tag reference in `.git/refs/tags/`, unless `-d/-l/-v` is given
+to delete, list or verify tags.
 
-Unless `-f` is given, the tag must not yet exist in
+Unless `-f` is given, the tag to be created must not yet exist in the
 `.git/refs/tags/` directory.
 
 If one of `-a`, `-s`, or `-u <key-id>` is passed, the command
-creates a 'tag' object, and requires the tag message.  Unless
+creates a 'tag' object, and requires a tag message.  Unless
 `-m <msg>` or `-F <file>` is given, an editor is started for the user to type
 in the tag message.
 
 If `-m <msg>` or `-F <file>` is given and `-a`, `-s`, and `-u <key-id>`
 are absent, `-a` is implied.
 
-Otherwise just the SHA1 object name of the commit object is
-written (i.e. a lightweight tag).
+Otherwise just a tag reference for the SHA1 object name of the commit object is
+created (i.e. a lightweight tag).
 
 A GnuPG signed tag object will be created when `-s` or `-u
 <key-id>` is used.  When `-u <key-id>` is not used, the