Clarify API microversion docs around handling 500 errors
Clarify API microversion docs around handling 500 errors

We shouldn't require a microversion bump for translating a 500 error to
some 400-level error code since 500s should not be part of the API
contract and clients shouldn't expect them for cases that the client can
change, i.e. don't ask for things that don't exist, don't ask to do
things that raise quote and you're already maxed out, etc.

There was some confusion in the doc about the statement that a
microversion is needed when changing from a 501 to a 400, but that's a
different case (going from something not being implemented to it
suddenly being implemented - that's a case where the client should
opt-in and a microversion bump is required).

Not returning 500s is just fixing bugs and shouldn't require a
microversion, the docs even already say that in the first footnote -
this just adds notes to clarify.

Also fixes the links to the [1] footnote.

Change-Id: I4526a72458a23662bd8aaa7f89be32844a511929

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