From: Chinmoy Chakraborty <chinmoy12c@gmail.com>

This commit lists the special strings used with `--date`
in `git-commit.txt` and also a brief explanation about
the strings in `date-formats.txt`.

Signed-off-by: Chinmoy Chakraborty <chinmoy12c@gmail.com>

Submitted-As: https://lore.kernel.org/git/pull.918.git.1616926790227.gitgitgadget@gmail.com