Fix MaxDepth problems introduced between 12.0.3 and 13.0.1

[mcon]

Fixes #2501 - have verified that the example provided in F# fails with no change and is resolved with the change.

Between 12.0.3 and 13.0.1 looks like _maxDepth = 64; was set in the JsonReader constructor - any inheritors which use this default constructor end up with a max depth of 64 as a result - from the existing tests on max depth, it seems that this setting is not actually required in order to ensure the max depth is respected.

I've also updated a few doc comments which erroneously claim the max depth as 128.

Update: as much as the initial fix (#2462) prevents a DOS attack, it does create other problems, such as 2501 - max depth is a bit tricky throughout the library, one option would be to make it mandatory on the JsonReader perhaps.

[Smaug123]

Suggest adding MaxDepth tests to verify that you can extend MaxDepth to be bigger than the default?

[JamesNK]

Thanks for looking at this.

I've fixed this in a different way here #2505