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>
---
 docs/devel/tcg-plugins.rst | 8 ++++++--
 1 file changed, 6 insertions(+), 2 deletions(-)

diff --git a/docs/devel/tcg-plugins.rst b/docs/devel/tcg-plugins.rst
index f93ef4fe52..be60b24566 100644
--- a/docs/devel/tcg-plugins.rst
+++ b/docs/devel/tcg-plugins.rst
@@ -27,13 +27,17 @@ Once built a program can be run with multiple plugins loaded each with
 their own arguments::
 
   $QEMU $OTHER_QEMU_ARGS \
-      -plugin tests/plugin/libhowvec.so,inline=on,count=hint \
-      -plugin tests/plugin/libhotblocks.so
+      -plugin contrib/plugin/libhowvec.so,inline=on,count=hint \
+      -plugin contrib/plugin/libhotblocks.so
 
 Arguments are plugin specific and can be used to modify their
 behaviour. In this case the howvec plugin is being asked to use inline
 ops to count and break down the hint instructions by type.
 
+QEMU also evaluates the environment variable ``QEMU_PLUGIN``::
+
+  QEMU_PLUGIN="file=contrib/plugin/libhowec.so,inline=on,count=hint" $QEMU
+
 Writing plugins
 ---------------
 
-- 
2.35.1