Fix some minor documentation issues with the state context draft extension

[robbert-vdh]

We could make these changes as part of the next merge, but it seems like less work to just do it now.

This changes a reference to set_state_context() to clap_plugin_state_context->set(), changes other references to function to use the same style, and adds a note that no call to clap_plugin_state_context->set() should be treated as CLAP_STATE_CONTEXT_PROJECT.

[CLAassistant]

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