git-prompt.sh: add missing information in comments

Mention that the command below is needed for prompt
in ZSH with PS1:
  setopt PROMPT_SUBST

Edit some parts that mention only the "current branch name"
being displayed in the prompt. Replace by mentioning that
the "repository status" is displayed.

Make it clear that colored prompt is only available
in PROMPT_COMMAND/precmd mode.

With-suggestions-by: SZEDER Gábor <szeder@ira.uka.de>