docs/devel: include the plugin API information from the headers

We have kerneldoc tags for the headers so we might as well extract
them into our developer documentation whilst we are at it.

Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Reviewed-by: Aaron Lindsay <aaron@os.amperecomputing.com>
Message-Id: <20210323165308.15244-3-alex.bennee@linaro.org>

docs/devel/tcg-plugins.rst

@@ -63,6 +63,11 @@ valid during the lifetime of the callback so it is important that any
 information that is needed is extracted during the callback and saved
 by the plugin.
 
+API
+===
+
+.. kernel-doc:: include/qemu/qemu-plugin.h
+
 Usage
 =====