docs/git-tag: explain lightweight versus annotated tags

Stress the difference between the two with a suggestion on
when the user should use one in place of the other.

Signed-off-by: Daniele Segato <daniele.segato@gmail.com>
Signed-off-by: Jeff King <peff@peff.net>
Reviewed-by: Jonathan Nieder <jrnieder@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Author: danielesegato

Documentation/git-tag.txt

@@ -42,6 +42,17 @@ committer identity for the current user is used to find the
 GnuPG key for signing. 	The configuration variable `gpg.program`
 is used to specify custom GnuPG binary.
 
+Tag objects (created with `-a`, `s`, or `-u`) are called "annotated"
+tags; they contain a creation date, the tagger name and e-mail, a
+tagging message, and an optional GnuPG signature. Whereas a
+"lightweight" tag is simply a name for an object (usually a commit
+object).
+
+Annotated tags are meant for release while lightweight tags are meant
+for private or temporary object labels. For this reason, some git
+commands for naming objects (like `git describe`) will ignore
+lightweight tags by default.
+
 
 OPTIONS
 -------