docs/tcg-plugins: document QEMU_PLUGIN behaviour

docs/tcg-plugins: document QEMU_PLUGIN behaviour

QEMU plugins can be loaded via command line arguments or via
the QEMU_PLUGIN environment variable. Currently, only the first method
is documented. Let's document QEMU_PLUGIN.

As drive-by cleanup, this patch fixes the path to the plugins
in the same section of the documentation.

Signed-off-by: Christoph Muellner <cmuellner@linux.com>
Message-Id: <20220316181412.1550044-1-cmuellner@linux.com>
[AJB: fixed some minor typos]
Signed-off-by: Alex Bennée <alex.bennee@linaro.org>