FindProtobuf: Add documentation for protobuf_generate()

Note, the argument `DESCRIPTORS` is not documented as upstream doesn't
provide this argument and code might break when switching to upstream
later.

Instead of the argument `DESCRIPTORS`
`PROTOC_OPTIONS "--descriptor_set_out=NAME.desc"` can be used.

Argument description has been copied from [1].

Fixes: #23037

[1] https://github.com/protocolbuffers/protobuf/blob/8ec0295ad7fc83d20609fcdf4830e3e3fddf2912/docs/cmake_protobuf_generate.md