[release-branch.go1] net/rpc: fix typo in documentation, clarify semantics of error...

[release-branch.go1] net/rpc: fix typo in documentation, clarify semantics of error return

««« backport ef713d7587f2
net/rpc: fix typo in documentation, clarify semantics of error return

Several of my students were confused by trying to use both the error
return and a reply return, so I figured it was worth explicitly clarifying
that returning an error overrides the reply.

R=golang-dev, r
CC=golang-dev
https://golang.org/cl/6327051

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