docs: Bring TableGen syntax a bit closer to reality.

It's not just def's but actually a limited subset of Object's that are
allowed inside a multiclass.

Spotted by Joel Jones.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@171935 91177308-0d34-0410-b5e6-96231b3b80d8
fix copy-paste-o

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@171936 91177308-0d34-0410-b5e6-96231b3b80d8