OGC API: Document the metadata item "oga_compliance_mode"

[jratike80]

Usage example in https://github.com/MapServer/MapServer/blob/809a0ad23a6bee437a715fae68e65bc83b674b48/msautotest/api/ogcapi.map#L63

What effect using compliance_mode have? It makes Mapserver to throw an error if OGCFeat request contains unknown parameters, aka vendor specific parameters. Otherwise they are silently skipped. The known parameters seem to come from a hardcoded list. Maybe Mapserver does not support "queryables" yet.

https://github.com/MapServer/MapServer/blob/main/src/mapogcapi.cpp#L1285