Document Checkpoints

[jmjatlanta]

Provides better documentation of checkpoints. No change in functionality

[DeckerSU]

Could you plz keep the original includes order and forward declarations in this PR. Leave only documentation additions. Also i would like to keep the following constructions:

#ifndef BITCOIN_CHECKPOINTS_H
#define BITCOIN_CHECKPOINTS_H
...
#endif // BITCOIN_CHECKPOINTS_H

Instead of just:

#pragma once

For the new source files i don't mind of #pragma once usage, but for sources inherited from Bitcoin, better to keep "existing order" of things. Just bcz it will help/promote to easy cherry-picking or merge some PRs from bitcoin upstream.

In other words i would like to keep document PRs - "document only". Such type of PRs should only add comments and don't touch includes, defines, pragmas, etc. IMHO.

[dimxy]

tbh i'd prefer to keep original files intact, if possible

[jmjatlanta]

I reverted the #pragma and removed the #include.

I understand an argument can be made to simply close this PR because it doesn't match the upstream Bitcoin repository's version. However, I believe there is value in documenting these things, and the forked code is so old that attempting to change upstream's version is futile and provides no benefit to the version we use.

However, I will happily accept any decision from the team.

[tonymorony]

these changes implemented in combined PR: #559