doc: exclude seastar::coroutine::internal namespace
doc: exclude seastar::coroutine::internal namespace

It is internal and just muddies the generated documentation.
Message-Id: <722e905f580374e3e3aa9fe510559bb7afb7d97f@scylladb.com>

avikivity/seastar,syuu1228/seastar,syuu1228/seastar,avikivity/seastar,scylladb/seastar,scylladb/seastar,scylladb/seastar,avikivity/seastar,syuu1228/seastar