grep docs: describe --recurse-submodules further and improve formatting a bit

Clarify that --recurse-submodules cannot be used together with --untracked,
and improve the formatting in a couple of places, to make it visually clear
that those are the commands or the names of configuration options.

While there, change a couple of "<tree>" placeholders to "_<tree>_", to help
with an ongoing translation improvement effort. [1]

[1] https://lore.kernel.org/git/CAPig+cQc8W4JOpB+TMP=czketU1U7wcY_x9bsP5T=3-XjGLhRQ@mail.gmail.com/

Signed-off-by: Dragan Simic <dsimic@manjaro.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

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