Note that Map fields cannot be optional

[rossng]

This tripped me up: the documentation explicitly says that Map fields cannot be repeated, but it doesn't mention optional.

When trying to compile protobufs with a map<string, Foo>`, I get the following error:

rpc.proto:121:19:syntax error: unexpected '<'
rpc.proto:121:48:syntax error: unexpected '>'

See also: protobufjs/protobuf.js#429

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[rossng]

I generally don't sign CLAs. This change is so trivial that it's not copyrightable IMO - feel free to rewrite it yourselves.