Documentation - add version info, misc fixes [ci skip]

[MSP-Greg]

Description

Recently spent some time with things other than Puma, came back to it, and...

There have been several additions in 5.0.0, so I added the YARD @version tag to all new methods/classes. Added a few @see tags, which link to other methods. The only actual code changes were a typo in two error messages that I did.

Left as separate commits. I can generate docs from this if anyone wants to see the changes...

Separate commits for clarity, obviously should be squashed.

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