protocol-native: Fix a crash

Without this check pa_source_output_set_volume() crashes if
so->volume_writable is false. The same check already exists for sink
inputs.

BugLink: https://bugs.freedesktop.org/show_bug.cgi?id=74529
source: Increase max outputs per source

This change is just for being symmetric with sinks, which got this
change in 26bf2df903b1c7439ad3eef0dc3bb1e777835cc3.
man: improve man page and help for cmdline options