docs/tcg-plugins: add L2 arguments to cache docs

cache plugin now allows optional L2 per-core cache emulation that can be
configured through plugin arguments, this commit adds this functionality
to the docs.

While I'm at it, I editted the bullet point for cache plugin to say:
    contrib/plugins/cache.c
instead of
    contrib/plugins/cache
to match other plugins.

Signed-off-by: Mahmoud Mandour <ma.mandourr@gmail.com>
Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Message-Id: <20210810134844.166490-6-ma.mandourr@gmail.com>
Message-Id: <20211026102234.3961636-18-alex.bennee@linaro.org>