diff: mark param1 and param2 as placeholders

diff: mark param1 and param2 as placeholders

Some l10n translators translated the parameters "files", "param1" and
"param2" in the following message:

    "synonym for --dirstat=files,param1,param2..."

Translating "param1" and "param2" is OK, but changing the parameter
"files" is wrong. The parameters that are not meant to be used verbatim
should be marked as placeholders, but the verbatim parameter not marked
as a placeholder should be left as is.

This change is a complement for commit 51e846e673 (doc: enforce
placeholders in documentation, 2023-12-25).

With the help of Jean-Noël,some parameter combinations in one
placeholder (e.g. "<param1,param2>...") are splited into seperate
placeholders.

Helped-by: Jean-Noël Avila <jn.avila@free.fr>
Signed-off-by: Jiang Xin <worldhello.net@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>