rpc: improve submitpackage documentation and other improvements

[stickies-v]

submitpackage requires the package to be topologically sorted with the child being the last element in the array, but this is not documented in the RPC method or the error messages.

Also sneaking in some other minor improvements that I found while going through the code:

• Informing the user that package needs to be an array of length between 1 and MAX_PACKAGE_COUNT is confusing when IsChildWithPackage() requires that the package size >= 2. Remove this check to avoid code duplication and sending a confusing error message.
• fixups to the submitpackage examples

[DrahtBot]

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Type	Reviewers
ACK	fjahr, instagibbs, glozow, achow101

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[stickies-v]

Rebased to master to make CI green, no changes otherwise. Thanks for the review @glozow, addressed all comments.

[fjahr]

utACK 5a7a720

This is ok to merge as is but if you decide to address 9deb910#r1476296848 I will quickly re-review.

[stickies-v]

Force pushed to incorporate the approach suggested in #29292 (comment)

Apologies for the slow follow-up while I was on holidays, thank you for the review!

[fjahr]

re-ACK 311f523

Apologies for even slower re-review :)

[stickies-v]

Force pushed to address @instagibbs's review, only very minor changes.

[fjahr]

re-ACK 78e52f6

Trial diff to check: https://github.com/bitcoin/bitcoin/compare/311f52371f37123f1f6186ac818db0741f643aed..78e52f663f3e3ac86260b913dad777fd7218f210

[instagibbs]

ACK 78e52f6

only the suggested changes, verified via git range-diff master 311f523 78e52f6

[glozow]

utACK 78e52f6

[achow101]

ACK 78e52f6