Add comments to struct cgraph_thunk_info

Add comments to struct cgraph_thunk_info

This commit adds comments to fields in the cgraph_thunk_info structure
declaration from cgraph.h.  They will hopefully answer questions that
people like myself can ask while discovering the thunk machinery.  I
also made an assertion stricter in cgraph_node::create_thunk.

Bootsrapped and regtested on x86_64-linux.

gcc/

	* cgraph.h (cgraph_thunk_info): Add comments.
	* cgraph.c (cgraph_node::create_thunk): Adjust comment, make
	assert for VIRTUAL_* arguments stricter.

From-SVN: r252828