[DOC BUG] git tag uses GIT_COMMITTER_DATE, not GIT_AUTHOR_DATE

In Git v1.5.4.1, Documentation/git-tag.txt advises:

To set the date used in future tag objects, set the environment
variable GIT_AUTHOR_DATE to one or more of the date and time.  The
date and time can be specified in a number of ways; the most common
is "YYYY-MM-DD HH:MM".

But this does not work because git tag uses GIT_COMMITTER_DATE instead.
In builtin-tag.c:

	header_len = snprintf(header_buf, sizeof(header_buf),
			  "object %s\n"
			  "type %s\n"
			  "tag %s\n"
			  "tagger %s\n\n",
			  sha1_to_hex(object),
			  typename(type),
			  tag,
			  git_committer_info(IDENT_ERROR_ON_NO_NAME));

i.e. it does not call git_author_info.

The backdating section was added to git-tag.txt in 5040beff,
and builtin-tag.c already used git_committer_info at that time.
The older git-tag.sh used tagger=$(git-var GIT_COMMITTER_IDENT).
Thus it seems the bug is in the documentation, not in the code.