Fix a few "it's -> its" typos
doc: Use correct parameter name wait_for_accept in documentation of pa_threaded_mainloop_signal()

also sneaks in a full stop in documentation elsewhere

Signed-off-by: Peter Meerwald <pmeerw@pmeerw.net>
core: add missing SET_PORT_LATENCY_OFFSET to dispatcher opcode string table

Signed-off-by: Peter Meerwald <pmeerw@pmeerw.net>
doc: Use \since doxygen command correctly in mainloop-api.h

Signed-off-by: Peter Meerwald <pmeerw@pmeerw.net>